[john]

Usually the rotors need to be machined if there is runout on them. This is sometimes easy to detect when driving a car as the brakes will seem to shudder as the rotor is not as thick all the way around. If the rotors are off, measure them with a micrometer (assuming disk brakes) at several different points to see if runout is present (numbers are not equal). As stated above, turning (or machining) the rotors is the most economically sound option as it only costs $10/rotor at checker. There is no need to replace them unless they are under the minimum thickness spec. Checker can measure them and check them if you have no idea what you are doing at all. They would then sell you new rotors of turn them.