I once carried my Domke F2 "backwards" for a day just to see if it was any better. It certainly made access easier but I rationalized that if I fell or the bag was stripped off my shoulder the contents would spill out easier than if even an unclipped cover was covering it. But one day does not a valid test make; I use it in the usual fashion and swear occasionally. I'm more concerned about the mystical affinity my camera straps have for the handle of the parking brake and seat adjustment levers in just about any car I've even owned.
I have A Domke F2 and the Domke US postal Service shoulder pad, and has never slipped off my shoulder.