Well just don't brush your teeth, think of all the water saved over the years (as well as toothpaste and toothbrushes).
I turn it on, get the brush wet, turn it off, put toothpaste on, brush, spit, mouthwash, spit then rinse the sink and toothbrush.
I never thought of leaving the water on.
As for the temp, get a single handle faucet. Love it in my shower, I just pull it on and it is at the last temp I had it at, no fiddling with it to get it right.