This is a very important point. Not so much about the mass pulling (pull what you are comfortable with, and the amount you'll feel okay pulling will naturally increase as you learn to understand your abilities) but about working as a unit with your healer. It's not the job of a tank to expect the healer to just heal anything and everything regardless of the situation. It's not the fault of the healer if they have no healing tools left and the tank just keeps pulling and pulling or getting hit by things they could avoid/mitigate. It's also often not necessary for the healer to keep the tank at 100% HP at all times as long as both are prepared for what's coming next. The healer and the tank have to work together and have to rely on each other.