This should be in the never ending debate section, arguements are, Ground up, if something falls, it will hit the ground and reduce chances of a short, other side, ground down, if you step on the cord or pull it, often with downward pressure you may break or partially break the connection to the hot lead, but the ground will still be making contact, Meh i dunno, I always install them down, I think they look better.
Next great debate, the screws for the cover plates, leave them however? all vertical? all horizontal? my eye picks them up when they are just wherever, and had Inspectors tell me when they see that, they take a closer look at the work as its usually a hack or amature job, but the guy ( or gal ) that takes time to make them all vertical likely took the time to do the job right and thus no need to pull things apart to inspect them.