nowadays, rotors are cheap enough, I'd just replace both because you really don't want 1 rotor that's new and one that's not. reason is the older rotor let's say you had machined.. well they took surface off, so now you have a "thinner" rotor, and overtime it won't be able to handle the heat and will warp faster, and won't give good braking power to that side. Not right away, but it will happen. then you'll be in that loop of replacing that one, and the one you replaced now will be the in same spot as the other.
Unless you're tight on money, I'd replace both and know you have new rotors and pads and won't have any problems with brakes.
A lot of shops I know refuse to turn rotors at all, they replace them with new iron. Eliminates comebacks and complaints of warpage.
