I always do nothing but heal, up to the first boss of any dungeon. That gives me time to check if the Tank is capable of holding aggro, whether the DPS are good enough to not steal aggro from the tank, and most importantly, if the tank pops his cooldowns so he can survive for more than 2 seconds without healing.

Only if all 3 conditions are met will I strap on Cleric mode and start dishing damage.

Sad thing is, in the last 5 dungeons I've done only 1 run has seen me dealing damage. They were all levelling dungeons, but you'd think that by DD or AV, the tank would know what the defensive abilities are for. Healers can't heal through everything.