Which way is up? (electric outlet)
The ground pin is usually facing down, and, if I recall correctly, when I installed some tamper-proof receptacles recently, the printing on them was right-side-up with the ground pin down.  I believe it's also a requirement in some localities.

On the other hand, people supposedly install the ground pin facing up as a safety precaution: if you're falling or dropping something onto a plug that's not securely plugged in, you'll hit the ground pin first so there's no risk of a shock.  If you've got both orientations within any one room, it's possible the installer didn't know what he was doing.

If your locality doesn't have a requirement for them to be facing down, I would just pick the orientation that you prefer and re-install the ones that are in the other orientation to match.