You start a dungeon with all your cooldowns available, so your first pull should be a wall to wall. Even when the healer can't keep up you should have enough tools available in this first pull to stay alive. Examine the situation. How many cooldowns did you use? How fast died the enemies? Was the healer stressed out or was he dpsing all along? Depending on the performance of your party you can decide how big the next pulls should be.