3 or 5 channel amps but using only 2 channels?

I have read and heard that an amp should not be operated without a "load" (speakers attached).

Does this apply to a 3 or 5 channel amp being used as a two channel amp? That is two channels have an input and speakers connected but the other channel(s) do not have either input cables or speakers connected.

OK to do or will it damage the amp in some fashion? Does this apply to tubes and solid state?
As long as a channel has no input, it matters not that it has no output. This covers 99.99% of the SS amps available.
I have a 5 channel amp and only run 2 channels for the fronts. No Problems.