Rotors are all metal, so I’m not sure what you’re asking.
If you replace rotors, and then turn them, what you are doing is shaving perfectly good metal off of a brand new part and therefore significantly shortening its life.
If you turn rotors without replacing them, what you are doing is making the rotor thinner and therefore more prone to warp, which is why you turned them in the first place.
In short: I just replace rotors. They’re cheap, and it lets me go longer before I have to fool with the brakes again.