Rotors often warp because someone "repairs" your car and then proceeds to put the wheels back on with the nearest impact wrench. This either applies too much torque to the nut, applies torque unevenly to different nuts, or both. Wheels MUST be bolted down in a criss-cross pattern using a torque wrench. Final torque is probably around 75-80ft/lb, but work your way up from 20 to 40 to 60, etc. I would check with your manual to be sure of final torque specs. Do it yourself too, because I have actaully seen people who didn't know how to use a torque wrench, so even if they say they used a torque wrench, be suspicious.
g
Greg