I would highly doubt you can warp a rotor from incorrectly tightening the lugs. You might damage your wheels, but the worst you could do to the rotor/hub is strip the threads or snap one off.
Rotor warping comes from heating the rotors beyond their limits to dissipate heat. The rotor then cools unevenly and "bends" the metal. On most cars you see it happen to the front rotors because they take most of the load. If your rotors are newer and haven't been worn too much you can have them machined back into spec -- otherwise I would invest in better quality rotors.