If you use a dial indicator you can turn a rotor on vehicle and see the runout .
I have never
found this to be true...
I don't know how may 100's of brake jobs I've done, how many rotors I've cut. The reason the lathe doesn't hit "true" is the "Micky Mouse" way the rotor is held.
After a while, EVERY car I saw with a pulse I tried the dial indicator run-out test on. If they were 2-sided, finned rotors I would cut just the one with the largest run-out and you know what?
Most times the pulse remained. Sure, there was run-out but, it wasn't the cause of the pulse, it was the rotor surface (and, of course, cutting it also gave it a new surface so it DID solve the problem)
I'm telling ya, modern rotor warpage is a myth... I'm sure, at one time, those crummy foreign car single disks got hot and could warp. And that was the start of the whole "rotor warp" sales pitch
I agree, who would cut a rotor today, I just replace them
BUT if you really want to know, check the run-out, find the worst one and replace just it and see if was the cause (Of Course, there IS a 25% chance that that rotor could also be the "Bad Surface" one too)
MythBusters should do a test on rotors! Let's ask 'em