Is It Safe to Warm Up Power Amps Without Speakers Attached

A friend brings his amp over to try with my speakers. We listen to my amps first. He plugs in his amp and powers it on to warm up for an hour before we power down my amps and move the speaker cables to his amp. Is this potentially harmful to his amp? I recall reading in a manual or an online post that someone suggested never running an amp without a speaker load. Does it depend on ss vs. tube...SET or push-pull...etc.? Cheers,