PaulBags :
Why'd my commented get deleted? "This is a passively-cooled PSU, so you should install it with the top exhaust grille facing downward." is totally wrong advice, if you don't believe me read the warnings in the picture YOU took.
I think your post got lost(forum bug maybe), not removed. I would see if it was removed and it was not.
I think this would depend on the case you are using to be honest.
System air will cool it.
If the power supply is at the top of your system putting its vent up will cause heat to have no place to go in many cases. Power supplies in the bottom of the case would be better served with the power supply vent face up.
The power supply has software to let you see the temperatures anyway so you can test. Tom's tests in a hotbox so it is not an actual case.
http://www.tomshardware.com/reviews/how-we-test-psu,4042.html