The biggest advantage I can see to looking over the right shoulder is control. When I look over my left shoulder, I have a tendency to drift left. This could put me out in traffic! If I look over my right shoulder, I suppose I would drift right. While that could put me into a ditch, that could be better than under a car

That being said, I look over my left for the reasons Chad cited.

I'm also planning to put a mirror on my bike one of these days