I wear a helmet mirror and wear headphones. In heavy traffic hearing what's behind you is pretty much impossible and I find the sound of traffic to be very distracting I'm talking about LA, Hollywood rush hour traffic. I know it's against the law to ride with headphones and I really don't care. When they start giving tickets to kids who blast their stereos so loud I can feel the bass blocks away, and to Harley riders who make their bikes obnoxiously loud, I might start to care.
As far as looking over your shoulder, just this morning I saw another guy using the look over the shoulder technique and he was weaving worse than a drunk guy driving a minivan full of kids while talking on his cell phone would.