I brush my teeth in the shower
Why do you feel that is acceptable?
Because it's more hygienic than brushing them in a sink.
It is smart to conserve water and multi task. But don't store your toothbrush in there.  You might get sick if you don't keep your tooth brush in a safe area from mold and bacteria.
I'm just kidding around.
Well, if you are able to brush your teeth while showering, and this doesn't end up using more water, you can keep doing it.  However, I just want to stress that you should keep your toothbrush in a holder on the counter, and not in the shower stall.  It is not safe to keep it in the warm, moist environment of the shower.