The reason for putting it ground up is that the vast majority of plug insertions occurs from above, with the plug angled down into the socket. This assures that the ground connector goes in first, as it's at the top, so you get the safety ground before any of the connections are made.
It's also a nice benefit that if something were to fall between the plug and the faceplate that it'd hit the ground and not short things (or short to the plate itself if it were metal).
This doesn't seem right to me. Given how the contacts are positioned deep in the socket, they wouldn't be reached if the plug isn't being inserted straight. Plus the ground pin is already slightly longer to mitigate this issue. Do you have a source for this explanation?
They are deep enough that it's not possible to make contact with the live and neutral pins before the ground pin. Try it, I just did and I wasn't able to do it regardless of angle.
