if your PSU is mounted at the bottom, your case has vents at the bottom where the PSU goes, and these vents are not blocked by the carpet or whatever, then have the fan closest to the floor. if it is bottom mounted and doesnt have all of that then have the fan facing inside the case
if the PSU is top mounted, the case has vents in the top where the PSU goes, and you wouldnt be tempted to cover up the vents with loads of stuff, then mount it at the top with the fan touching the top of the case. if not then mount it with the fan closest to the bottom of the case.
hopefully that will cover it (unless youve got one of those cases that mounts the PSU in a strange place)