Mounting a receptacle that takes a NEMA 5-15 or 5-20 upside down isn't supposed to change the polarity. No matter how it's installed the wide slot in the front is supposed to be neutral, the narrow hot and ground should always be ground even if it's on top.
I've heard folks suggest that mounting the ground up 'protects' things by making something dropped and hitting an exposed plug tab less dangerous since it hits the ground first. I think that's kind of reaching for a reason, myself.
I finished building a house a couple of years ago and am pissed at all the safety 'improvements' in the electrical. You simply can't get a plug into an outlet with the stupid safety shutters in the way, the 'arc fault' breakers on all the convenience outlets snap open for no reason at all and in retrospect I should have shot the electrician I fired from the job as a warning to other electricians in the field for all the crap work I found after he was shoved off the property.