I saw the ground on top in an industrial facility I was working at in North Carlolina for a couple of weeks. I asked around and nobody knew why. Otherwise, I see it on the bottom.
@carlsbad2 - From my reading long ago the issue may have to do with safety and the use of metal or plastic outlet covers. Plastic covers/plates can go any way but the convention is ground on the bottom.
The issue with metal covers is the possibility of them slipping off over time while a plug is still attached. If the ground is on the top is is less likely to cause a short. In this case when the cover slips it will HOPEFULLY encounter the ground pin first and stop. However if the H and N pins are on the top when the face plate falls off AND there is a plug there it could start some bad but intermittent shorts leading to fires.
I remember seeing a picture of this. The metal cover had 2 slots burned into it (looked like a saw kerf) where it had made contact with a plug and the shorts has burned away the plate. For whatever reason the shorts were not solid enough to actually trip the breaker, and who knows how long that plate had burned without being discovered.
In cases when there is a metal cover and the plug is horizontal neutral should be on top if possible.
Of course in a perfect world the plates never get jostled, the center screws never come loose and this is never a problem. I just installed an outlet under a sink and while it’s tight now who knows what 10 years of using that space for storage will do. This is even worse in an industrial setting.
I’m not actually an electrician, but in all my DIY electrical and home mechanical work there’s no flimsier screw than the little one in the center of an outlet that holds the plate on, and no piece of equipment gets more wear than an outlet cover. That’s why so many apartment outlets don’t even have covers or you’ll find them broken or loose. They get a lot more wear and tear than you’d think.
Also, for the home, stick to plastic outlet covers unless you know there’s high traffic/high movement. Your home audio gear is not going to be served by metal there.