Checking amps, without loads

I've seen warnings a few times, about not checking/testing stereo amplifiers without loads. Would this apply to instances both with, and without any input to the amp, or only with an input? To all amps, or only particular kinds? What happens? Transistors 'overrev'?

Thanks.

It was only a problem with valve amplifiers, or really cheap, nasty, and badly designed transistor ones (but it would be very rare!).

Without any input, it makes no difference anyway.