Depending on the current thickness of your rotor will really determine if the rotors can be machined. The tolerance BMW gives for the service life of the X5 rotors is not very much. More than likely your rotor thickenss minus the material that must be removed to provide a clean flat surface will be more than the service limit on the rotors. Sure, your car will stop fine in the beginning with new pads, but as the pads start wearing down, the caliper will be working that much further to make the pad contact the rotor.