I have done it for more than a year during summers. When my GPU fan stopped working, I've had big temperature problem (having 70`C+ temps). So I opened the PC Case from side and put a pedestal fan right next to it, so close that all of it touching the pc case (fan facing the motherboard) and it actually dropped 20`C of temperature right away through out summer.
Edit: I think I have to mention this also, one big drawback with doing that was dust accumulated really fast so for that I had to clean PC more often with a dust blower.