I see this all the time and the bigger issue is air flow around the receiver. If you have it 'open' all around the other sides, but then put it inside a cabinet with a closed door or something then you really don't have anything open and are definitely at risk for blowing the unit out.
But, maybe it is on a very open rack with a good ventilated room and then you could probably lay something across the top of it and it wouldn't hurt the receiver.
Solution: Use it, check it, then decide. It shouldn't be an issue or affect the receiver to run a little warm for a few days if it is an issue. But, after a particularly good movie, check all the openings for heat and see how it is doing.
I don't think a fan blowing through equipment is ever a bad idea really as long as there isn't to much dust with it.