I put my PSU on the bottom cause I had a Radiator on the top.
I then decided that my PSU (which has a top mounted fan) was a bit noisy so I flipped it over so that it ran through the grill. I was afraid this was to restrictive, so I cut back the grill to the edge of the PSU.
Now my PSU gets fresh air, and is silent, since it is close to the carpet.
Of course I put on larger non-lockable wheels to this case. So their is allot of clearance to pull air in.
Mike