If it was in the holster, most people don't teach that the muzzle direction rule applies (since in the holster it's supposed to be inert). A lot of carry methods have the muzzle pointing at things you would not want it pointing at when unholstered.

Yeah agreed. If people taught this than thigh holsters would be out (muzzle direction when sitting down), so would shoulder holsters. Pretty reasonable to expect a gun to not go off randomly, especially when reliable drop safety mechanisms have been around for a long time.

In general the safety rules I was taught were defense in depth -- multiple failures need to happen before someone can get hurt. So, even when you have a holstered firearm, pointing the muzzle away from people is preferable because then if it somehow does fail at least it's less likely to shoot someone.

Obviously that's not always practical, but if you are placing a chambered firearm down on a desk, you might as well try to point it in the direction of least harm, it basically costs zero to do.