Okay I feel like we should get an actual mathcraft on here. I was playing Jarvan against zimike's tank Sona (~250 armor) and was doing true damage to him, and he got so mad he allegedly quit Dominion (but I'm sure he'll be back to play Black Cleaver Lulu).
So uh, here's some numbers. Assuming 21 in offense and ArPen marks/quints.
Item specific notes:
0 Cleavers without LW:
100 armor: 100 * .88 * .92 - 24 = 57 armor = 36% damage reduction, from 50%.
150 armor: 150 * .88 * .92 - 24 = 97 armor = 49% damage reduction, from 60%.
200 armor: 200 * .88 * .92 - 24 = 138 armor = 58% damage reduction, from 67%.
250 armor: 250 * .88 * .92 - 24 = 178 armor = 64% damage reduction, from 71%.
300 armor: 300 * .88 * .92 - 24 = 219 armor = 69% damage reduction, from 75%.
Cost: 0
Other stats gained: Nothing
Overall damage increase: 14% to 6%.
0 Cleavers with LW: What Dominion used to mostly look like.
100 armor: 100 * .88 * .92 * .65 - 24 = 29 armor = 22% damage reduction, from 50%.
150 armor: 150 * .88 * .92 * .65 - 24 = 55 armor = 35% damage reduction, from 60%.
200 armor: 200 * .88 * .92 * .65 - 24 = 81 armor = 45% damage reduction, from 67%.
250 armor: 250 * .88 * .92 * .65 - 24 = 108 armor = 52% damage reduction, from 71%.
300 armor: 300 * .88 * .92 * .65 - 24 = 134 armor = 57% damage reduction, from 75%.
Cost: 2135
Other stats gained: +40 AD
Overall damage increase: 28% to 18%
1 Cleaver without LW:
100 armor: 100 - 30 = 70 * .88 * .92 - 39 = 18 armor = 15% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 - 39 = 46 armor = 32% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 - 39 = 74 armor = 43% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 - 39 = 103 armor = 51% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 - 39 = 131 armor = 57% damage reduction, from 75%.
Cost: 2865
Other stats gained: +250 Health, +50 AD, +10% CDR.
Overall damage increase: 35% to 18%
1 Cleaver with LW:
100 armor: 100 - 30 = 70 * .88 * .92 * .65 - 39 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 * .65 - 39 = 16 armor = 14% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 * .65 - 39 = 35 armor = 26% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 * .65 - 39 = 53 armor = 35% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 * .65 - 39 = 72 armor = 42% damage reduction, from 75%.
Cost: 5000
Other stats gained: +250 Health, +90 AD, +10% CDR.
Overall damage increase: 50% to 33%
2 Cleavers without LW:
100 armor: 100 - 30 = 70 * .88 * .92 - 54 = 3 armor = 3% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 - 54 = 31 armor = 24% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 - 54 = 59 armor = 37% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 - 54 = 88 armor = 47% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 - 54 = 116 armor = 54% damage reduction, from 75%.
Cost: 5730
Other stats gained: +500 Health, +100 AD, +20% CDR.
Overall damage increase: 47% to 21%
2 Cleavers with LW:
100 armor: 100 - 30 = 70 * .88 * .92 * .65 - 54 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 * .65 - 54 = 1 armor = 1% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 * .65 - 54 = 20 armor = 17% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 * .65 - 54 = 38 armor = 28% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 * .65 - 54 = 57 armor = 36% damage reduction, from 75%.
Cost: 7865
Other stats gained: +500 Health, +140 AD, +20% CDR.
Overall damage increase: 59% to 39%
3 Cleavers without LW:
100 armor: 100 - 30 = 70 * .88 * .92 - 69 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 - 69 = 16 armor = 14% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 - 69 = 44 armor = 31% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 - 69 = 73 armor = 42% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 - 69 = 101 armor = 50% damage reduction, from 75%.
Cost: 8595
Other stats gained: +750 Health, +150 AD, +30% CDR.
Overall damage increase: 50% to 25%
3 Cleavers with LW:
100 armor: 100 - 30 = 70 * .88 * .92 * .65 - 69 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 * .65 - 69 = 0 armor = 0% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 * .65 - 69 = 5 armor = 5% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 * .65 - 69 = 23 armor = 19% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 * .65 - 69 = 42 armor = 30% damage reduction, from 75%.
Cost: 10730
Other stats gained: +750 Health, +190 AD, +30% CDR.
Overall damage increase: 62% to 45%
4 Cleavers without LW: "The best build in Proving Grounds."
100 armor: 100 - 30 = 70 * .88 * .92 - 84 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 - 84 = 1 armor = 1% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 - 84 = 29 armor = 22% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 - 84 = 58 armor = 37% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 - 84 = 86 armor = 46% damage reduction, from 75%.
Cost: 11460
Other stats gained: +1000 Health, +200 AD, +40% CDR.
Overall damage increase: 59% to 29%

Aparkhurst last seen exploding with anger.
4 Cleavers with LW: "The best build in the UNIVERSE!"
100 armor: 100 - 30 = 70 * .88 * .92 * .65 - 84 = 0 armor = 0% damage reduction, from 50%.
150 armor: 150 - 45 = 105 * .88 * .92 * .65 - 84 = 0 armor = 0% damage reduction, from 60%.
200 armor: 200 - 60 = 140 * .88 * .92 * .65 - 84 = 0 armor = 0% damage reduction, from 67%.
250 armor: 250 - 75 = 175 * .88 * .92 * .65 - 84 = 8 armor = 7% damage reduction, from 71%.
300 armor: 300 - 90 = 210 * .88 * .92 * .65 - 84 = 27 armor = 21% damage reduction, from 75%.
Cost: 13595
Other stats gained: +1000 Health, +240 AD, +40% CDR.
Overall damage increase: 67% to 54%

DAT ASHE.
--------------------------------------------------
Champion specific notes:
25% penetration
Champions: Darius
70 armor (after BC reduction) becomes 43 without LW, and 28 with, prior to flat penetration from # of BCs.
105 armor becomes 64 without LW and 44 with.
140 armor becomes 85 without LW and 55 with.
175 armor becomes 106 without LW and 69 with.
210 armor becomes 128 without LW and 83 with. Therefore, Darius can deal true damage attacks to targets with 300 armor.
15% reduction
Champions: Kayle (at 5 hits), Shyvana, Xin Zhao
100 armor becomes 85 before rest of calculations above.
150→128
200→170
250→213
300→255
350→298
400→340
20% reduction
Champions: Urgot
100 armor becomes 80 before rest of calculations above.
150→120
200→160
250→200
300→240
350→280
400→320
25% reduction
Champions: Jarvan (effectively), Jayce, Trundle (at 16)
100 armor becomes 75 before rest of calculations above.
150→113
200→150
250→188
300→225
350→263
400→300
30% reduction
Champions: Wukong
100 armor becomes 70 before rest of calculations above.
150→105
200→140
250→190
300→210
350→245
400→280
35% reduction
Champions: Renekton
100 armor becomes 65 before rest of calculations above.
150→98
200→130
250→163
300→195
350→228
400→260
450→293
40% reduction
Champions: Nidalee
100 armor becomes 60 before rest of calculations above.
150→90
200→120
250→150
300→180
350→210
400→240
450→270
500→300
So, the synopsis of this is that if you're gunning for pure offense, get a LW after your first BC unless you have % reductions/penetrations from other sources, in which case you'll probably want 2 BCs first or possibly 3 depending on how much from the other sources you have (your allies might also contribute theirs). This is good to know, because usually you need some kind of life steal to keep the true damage rolling in throughout the game.
So... yeah. Inb4 the item gets removed in 5 minutes making this thread useless.
Joke build, it is not. Psssst Xypherous.