I see that new power supplies are coming with the fan at the top/bottom. If you are replacing an old one with the fan @ the back does the fan face up or down? If it faces up there are no vent. holes in the case. if it faces down it is going to blow the hot air into the case.
Most modern cases have a grill above the PSU bay so the problem that you speak of does not arise. In my latest build the PSU sits at the bottom of the case and has a removable grill for cleaning under the PSU bay.