I modeled locally for a few years in the 1980s, and the agency would tell us to ALWAYS carry our shoulder bags over our left shoulders, never right. All these years later, I still carry on my left and wore my college backpacks and Prada backpack slung over my left shoulder if I was using just one strap. I don't know where this left-shoulder edict began or why; maybe there was a desire for consistency in the way in which models carried their bags during our shows.