FWIW, I almost always use tap water for brushing teeth and rinsing my toothbrush, even in most Tier 3 places, and certainly wouldn't bother in a Tier 1 city. I've got a bit of an iron stomach, though, and also used the tap water in places like Bangladesh with no ill effects...

This is one of those things where you can quickly drive yourself mad if you worry about it too much: i.e. what about accidentally swallowing a bit of water in the shower? Or maybe that restaurant washed the dishes in tap water and some got left behind? I'd be more concerned about the dangers of Chinese traffic and the chronic "fog" that engulfs most cities than any bacteria in tap water.