It's dangerous to throw that out when it comes to stateless groups - if you treat their people inhumanely, they use it for recruiting purposes. Abu Ghraib and Guantanamo are cases in point.
But people prone to savage acts (which can be almost anyone given the right circumstances) are going to claim that the Other are doing horrible things regardless of whether it's true. Saying anything necessary to rile your people up against the enemy has been an everpresent tactic since, well, whenever human beings were first able to muster up the strength to attack the other guys. European Jews didn't need to actually drink the blood of a single Gentile baby to become hated and feared.
It wouldn't matter if America gave every prisoner in Guantanamo a massage and ice cream sundae every day, Al Queda would still say they were skinning them alive and drinking their blood. No doubt the reverse is true to an extent as well. In fact, I may be doing it here. Hm.
Propaganda will be promulgated regardless of the facts. I'm not saying for one second that we might as well do our worst anyway, but there's no reason to expect the enemy to appreciate your restraint. We won't be hearing a Taliban representative say, "You know, Americans and Israelis aren't so bad after all" anytime soon.