Current wisdom says it should be on the bottom (toward the processor), but if there is a bit of room at the top of the case, I like to face mine upwards. Three plusses about that: 1-it will be drawing some of the warmest air from the case, 2-it won't be competing with the CPU fan and exhaust fan for air and 3-the main PCB in the PSU will be on the bottom which is its 'natural' position.
Some cases have a reversible PSU mounting plate just for that purpose, while other cases will have to be modded a bit. Have nibbling tool, will mod... 😉
My current PSU is a server grade Enhance unit that HAD two fans in it. The top fan WAS an 80x15 and the rear fan WAS an 80x25 and they were too noisy either together or separately. So I pulled them both. Put a venturi from a junk fan in the rear as a place holder (never throw away a dead or noisy fan - cut the leads for your wire collection and cut the motor out of it as you may be able to use the venturi for a spacer one day) and blocked the top fan hole. Slapped my old "outside the box" mod on it and it now intakes mostly from the rear and a little bit from the bottom. So it's sort of a compromise. But the PSU runs nice and cool and so does the rest of the system. YMMV.
.bh.
