The thing is most of PSU's are designed to Suck air in from below. The idea of a bottom mount PSU is that suck air in from under the case and blows it out the back so its not even sucking in air from the case. If it was to be mounted at the top it would face the same way. Fan down, just it would suck in the hot air from inside the case.
And if for some reason the fan was on top most cases allow you to remove the mounting bracket for the PSU and allow you to do a 180 flip on the PSU. Most PSU's only go in one way though but again most should allow you to do a 180 on the mounting bracket.
Also what case do you have?