Re: Not just an angry American
USAian here. "Ground down" has been the standard, ever since those things were introduced (I know, I was there).
A few years ago, some bright light got the idea that they should be installed "ground up". This was post 1993, when my house was built, but I can't nail it down any more precisely than that.
The word-of-mouth reasoning for "ground up" is that a wire, metal cover plate, or other conductor, falling across a partially unplugged connector, would contact the ground first, instead of falling across the two line terminals, as it might, if the outlet had been installed "ground down".
This seems to me to be a low probability occurrence (to say the least) and the National Electrical Code agrees with me as it doesn't state a preferred orientation (and shows outlets "ground down").
So "down" is "normal", unless you're paraoid, then "up" is OK. It seems to be an individual preference or the preference of an overly controlling local electrical inspector.
A secondary reason is that some electricians mount outlets controlled by wall switches in "ground up" configuration to distinguish them from unswitched outlets. This seems to be an uncommonly sensible idea.
// yeah, I know, way TMI...have a nice weekend!