Assuming your PSU has a fan and that your case has ventilation holes in the bottom of the case, your PSU should face down (fan facing down) for the fan to intake cool air from underneath and cool your PSU.
Assuming your PSU has a fan and that your case has ventilation holes in the bottom of the case, your PSU should face down (fan facing down) for the fan to intake cool air from underneath and cool your PSU.
Really makes no difference, personally I like to mount my psu facing down for cosmetic reasons, and because my case has a psu dust filter so why not use it.