My personal preference has always been ground-down, for anthropomorphic reasons. A NEMA 5-15R looks like a face. It just looks like it "wants" to be ground-down!
I have seen a few fridges, and once owned one, with a ground-up cord. So I installed that receptacle ground-up. I flipped it when I bought a new fridge.
Electric ranges/dryers nearly always have ground-up cords, so I always install NEMA 14 receptacles ground-up, to avoid stressing the connection.
Article 551 illustrates receptacle configurations for RV parks, and shows all of them ground-up. However, I don't see a requirement for such an installation. This is the closest the NEC gets to taking a side on the issue.
