Personally I would aim the fan up, and that's what I've done with the last 5 machines I built with Antec 300 cases. These cases are vented pretty well and I add fans in the front too, so if anything it would blow air through the PSU and out the back.
I think floor mounted or desk mounted is also important. Downward facing vents can get blocked by carpet and possibly clog from dust faster than the rest of the case. I would look at the case you have and determine which orientation will allow the PSU fan to get the most air and do it that way.