All things being equal, I don't see how the "view" should be any different in scope. Traffic activity is almost always to your left, however, and I don't see any advantage to putting the back of your head towards it by looking over the right shoulder. It seems like a much better habit to look left, that way, even if you're looking back, if something is happening on the road ahead, you have a chance to be alerted to it through peripheral vision and during the transition from straight ahead viewing to rearward and back. It's fractions of a second, but still...