It depends on what you are doing, given that you are doing AK you were playing exactly as you should. It's a farm run, so as a healer you are most efficient while doing DPS rather than keeping tank topped off. As long as the tank is not dead you are doing your job right. Not topping off is only an issue if your group is struggling to begin with, then as a tank you are not sure when to use your cooldowns as you don't know what your healer is doing.