If your just changing your pads and before changing them the brakes work smoothly with out pulsing then your rotors are probably true so sanding is all you need to do to lightly rough the surface to help bed in the new pads.

If your brakes are pulsing when you go to change the pads then its a good idea to have the rotors turned to make sure that they are true. If the rotors are turned then there's no need to sand them becasue the machining will leave a surface sufficently rough to bed in the new pads.

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