in my experience/experiment turning rotors for my minivan, i don't use a brake lathe. i made a fixture that fully supports the hub area as it mounts on the hub.
provided the rotor is thick enough, and not full of rust holes on the back side, i first run them through the blaster cabinet. mount on the fixture, then mount the fixture in my lathe. i have found it usually takes only .004-.005 to true up, using a fairly slow rpm and feed rate on the lathe.
i have zero problems doing this until i reach the low limit on the rotors.
using mid grade pads, this experiment has gone tens and tens of thousands of miles, never once experiencing warped rotors, but i don't run the pads down to metal on metal contact either.

now with that said, it is a waste of time and money doing this when rotors are so cheap ! also, using a "real" lathe and the correct feed and speed, i feel you get a MUCH better surface on the rotor than the brake rotor machines in use. couple that with a proper bed in procedure, and warped rotors are a thing of the past.
this is just my experiment, i don't recommend you try it. [and most do not have a lathe at home anyway] i did this just to "see what happens".