Originally Posted by Sharag
In Varus's case, there is his W, Blighted Quiver which is optimal when you use an ability after every 3 AA's. So in a way, it's compelling you to save your abilities so you can trigger the extra damage.
On the other hand, his abilities are not very gratifying if you do that and are often better used BEFORE you have applied stacks. (to poke with q, to slow and reduce healing with E or to initiate with R)
So in the end, it's better to play Varus as if he doesn't even have that W and just consider yourself lucky whenever you do manage to get the proc damage from blight.
In doing this, I feel you neglect the role in which Varus is made to serve: Damage. If you play to blow your stacks efficiently, you will notice the damage you deal actually spikes considerably, still being noticable, if not valuable, even at 35-40 minutes in, when you've reached your core build + 2 or 3 items.
The argument to use Varus's E quickly to deter advancing enemies is one that many AD carries will sympathize with, but in frantically fending off threats, it's easy to forget that in most cases, the time cost of putting out 2 auto-attacks before you E is very small when you have your phantom dancer. The enemy may advance towards you in the ~1.5-1.6s it takes to shoot twice, but you have allies to defend you with stuns, roots, slows, and plenty of damage to stack onto yours. If you don't, then what are you doing sticking your neck out to danger alone?
And if you're wondering, I cite the strike number of 2 before pressing E in time-sensitive situations because that generally is the break-even point of value between damage and safety. If you're willing to take calculated risks, you may even find it worth it to take hits to bring out even more blight damage. Just as Varus bares his body to the world, so should you show your courage to your enemies. Out of most of the AD carries in the game, Varus benefits a great deal more from risky plays. Just don't forget that risks should always be calculated. Your time to man up and stand against Jarvan should not be when he still has his ult and enough health to trade bursts.
I'd also like to weigh in my input to the debate of maxing E first over other abilities by saying that I prefer to max W first in most situations. 2 auto-attacks plus E means 40-50% of that Sona/Ezreal/Soraka/Corki/etc.'s health and an easy disengage. With the attack speed buff from Varus passive (off of minion kill, it's useful more often than you think), 2 auto-attacks is the amount you can squeeze in before a reaction if you surprise the enemy (catch that Ezreal hitting your caster minion). Max W when you favor opportunistic poke-bursts or trades in lane, which is the way bottom lane swings most of the time. Max E when you favor strong engagements or quick, coordinated burst with your support; usually only the case with an all-in like Leona, Blitz, Ali, and sometimes Taric.
Of course, you may feel free to take everything I say with a grain of salt. I am by no means an expert, but if I were to pick my favorite ADCs, it'd probably be Ashe and Varus, because their position play is more engaging and their potential unfolds in more interesting manners.