Which guns would that be? I don't know of any that the slide can be held back all the way without emptying the chamber.
Of course. But if you don't pull the slide all the way back, you can reveal the chamber without ejecting a bullet (or chambering a bullet, if the magazine is in). For instance, if it is your practice to keep the chamber empty, to reduce the possibility of an accidental discharge, you would clear the weapon, insert a magazine, and pull the slide back just enough to confirm that the chamber was empty, and let the slide back gently. Then you're good to (not) go.