In the US, most 15 and 20 AMP receptacles are installed with the hot and neutral on the top and the ground below. However, sometimes they are installed the other way around, with the ground above.

I have heard vague explanations of safety given for this - eg, plugs are more secure in the upside-down position or a wire falling from above that lands on a ground is essentially harmless whereas landing across the hot and neutral results in an extremely dangerous short.

Do these explanations hold water? Are there other reasons? Is it just superstition?