Backdooring is completely valid and can put a LOT of pressure on the enemy team, but it has its own rules for Dominion.
1: account for the enemy: Make sure you know where they are. Visible defending top from your team pushing lane, bot, or dead. Any left roaming only limits your options for escape.
2: Put significant pressure on the enemy. A good eve, leb, trist, teemo, etc. can put a lot of pressure on decisions for who to send down. They pose enough of a threat to burst or win a 1v1 with a carry under most circumstances, if played right. This should force them to send a bruiser or tank down, or possibly 2 champs, removing valuable utility from top and allowing your team to make a good push and win the fight.
3: Backdooring is not a sacrifice play (but it can lead to one). If you neutralize a point just to have a respawning champ kill you and take it right back, you accomplish nothing. If you do not put enough pressure for your team to win top, you accomplish nothing. Be prepared to harass the enemy team as much as possible. Singed, Rammus, even a clever Teemo, Eve, or Shaco (I will not speak of the currently broken flashspammer) can distract the enemy for a long time, and put up an amazing chase, wasting valuable time for the enemy. If they get smart and abandon the chase, you are in prime position to make a big play, ganking bot, harassing mid some more, or setting up a team play. The longer you distract them, the better. If they spend 30 seconds having 3 champs chase singed, you may end up with a dead singed, but you gain 30 seconds of a 2 champ advantage for your team, and still force those 3 to recall afterwards.