Stacy @ Bolty.net has a great post about keeping an eye on traffic behind you while stopped. If you haven't checked out her blog, you really should. You'll learn something.
I've been bumped in my car while at stop signs and red lights so, I've let myself become a bit paranoid about it. I also keep my bike in gear while stopped, just like keeping a round in the chamber. You think you can react fast enough but chances are you can't.
Now, to be honest, there are times when I'll stay in neutral if I can see no one is coming up behind me. I live in Indiana, we have plenty of long flat stretches of road, so this is pretty easy to do.