electrical engineering in the comments

1. Assume a three-prong plug is inserted into a NEMA 5-15R or 5-20R duplex outlet, housed in a metal workbox & metal faceplate. Years down the road, due to vibration, poor installation, whatever, the faceplate becomes loose and falls off. Hopefully the dislodged plate will not contact ANY of the prongs, because the plug “should be” fully inserted. But if it does, and the ground pin is on top, gravity will help assure that it’ll contact the ground pin only and/or first. That “should” minimize risk of shock, even though the outlet box is in a “failed” state, and the flat on the ground prong “should” assist with holding the fallen faceplate somewhat level. Also, with a path to ground (since the faceplate is now dangling by the ground prong), even if the faceplate contacts the line/L, it “should” open the breaker. Yes, these are all theoretical, which is why the word “should” is in double-quotes, but I’ve fiddled with outlets, metal workboxes, EMT, etc., long enough to see the possible benefit.
    
2. Most commercial/industrial equipment, which is what I use in my lab, assumes a ground-up outlet. This is potentially a pain when you have a molded plug where the cable is turned 90° (so it runs against the wall), and it would’ve run down (toward the floor, at about a 45° angle [from vertical]) in a ground-pin-up outlet – or it’s simply supposed to run straight down to the floor – but you’re stuck using a ground-pin-down outlet. Not a huge pain, but when you’re already running dozens of instruments on a bench, and there’s nearly a mile of cable around….
    
3. It’s a stupid reason, I admit, but I’ve found with the outlets I prefer (Leviton 20-A commercial grade, e.g. #R62-CBR20-00W), and the typical EMT routing/orientation I use, it’s MUCH easier to stuff the box (fold the wires in, after connection to the outlet) if the ground pin is up. I know, I know: “cry me a river”. 😜
    
    
   ALL THAT SAID, in a residential setting, you’re probably better off with the ground pin DOWN, because most consumer-grade electronics and accessories (extension cords, switched taps [so-called ‘power bars’], etc.) assume that’s the way your outlets are oriented. You typically don’t see metal faceplates in a residential setting these days either, outside of some basements. They’re almost exclusively insulating, non-combustible plastics now, like nylon, so the danger of the faceplate falling off and hitting a live prong is no longer present.
    
   Disclaimer: I am not an electrician by trade, though I’m fairly familiar with the NEC, performed industrial installations that have passed inspections, and my EE background should count for something (though I personally know EEs who can’t figure out how to check the oil level on their own car). tl;dr – caveat lector.