I've seen both too. If there's no grill on the bottom of the case then you have to mount it as you have. But i'm sure underneath the PSU will be a grill, for a WC rad/PSU, for it to inhale via the bottom of the case.
The idea is that the PSU breathes fresh air via the grill in the bottom of the case and exhausts it out the rear. Otherwise it will suck already warm air from inside the case.
Google for images
and you will see enthusiasts mount it the right way round, where they can.