It doesn't matter where the fan is, it's doing the same job because the PSU is effectively a sealed unit.
With the fan on the back it will be sucking in air from outside the case and blowing it out the bottom or front of the PSU into the computer case.
With the fan on the bottom it is blowing air out of the bottom of the PSU into the computer case, but that air is still being sucked in from the back of the machine.
It's pretty much like just having a fan right at the end of a tube or having one in the middle of a tube, the air will still come from the same place and go to the same place regardless of the position of the fan.