If you are having the case directly on top of a surface like carpet, then its best to have the unit mounted with fan facing up. Will save cleaning the dust all the time. Otherwise facing it down is the way to go (personal opinion). Also stops shit from falling in there when swapping out parts.
Down. Draws in cool air from outside of the case, instead of the warmer air from inside. Will prolong the life of your PSU. As long as it isnt on carpet.