I tend to mount mine (on those rare occasions that I actually use it) on the curb side for a couple of reasons:

1. I mount my bike from the left side, and it's easier to lean the bike into me while mounting the basket (Nashbar Townie). Downside is the lower hook is a PITA sometimes to mount because of the drive train.

2. If my bag should fall off, in theory my stuff has a reduced chance of being run over.