As there is no guidance in the matter in the NEC, I look at it from a marketing point of view. Here in Northern Illinois grounds are down or to the right. From a marketing point of view...imagine your favorite client paying you $20,000 for their major rewire...they have a grand party and you've installed the grounds up because you think it matters. All the guests are looking at the new remodeling job & saying..."He installed the outlets upside down...GREAT JOB!"
Having seen so many pictures of plugs plugged into receptacles with the ground pin hanging half way out of the slot, by the wieght on the plug, I would tend to think that ground down would be safer. Just my $0.02 worth.
The 2005 NEC Handbook addresses this in 406.4(D). Receptacles shall be flush or protrude out--- then in the Green Print(which is an explanation of the code) it says The NEC doesn't specify the position (blades up or down)of a common vertically mounted 15 or 20 amp receptacle. it shows pictures throughout mounted both ways. i would recommend checking the job specs if there are any.