I think it depends on the size of the tank. I have a 70G right next to a radiator (which in my view is probably cooler than right above it since the heat rises). It was the only place to put it, and I turned the radiator off permanently. In the dead of winter last year though I turned the radiator on because the room was fecking freezing, and monitored the tank closely. It took over an hour to have any effect at all, and never did the temperature rise too high, even after hours of the radiator being on. In fact, it did mean the heaters didn't have to work 100% of the time which they both practically were, and probably saved electricity.
If you're talking anywhere under 50G though, it's a really bad idea I reckon. If you aren't prepared to be very diligent when the heating is on, have total control over when your heating is on and how high it is on, and be prepared to move the tank or never use the radiator again, I wouldn't even try it. You could very easily fry your fish.
