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!"
Dave