There are two conflicting points of view on this issue. Some manufacturers say it is not necessary because their drums and rotors are finished at the factory and come ready to install. Resurfacing is therefore totally unnecessary. Furthermore, it would reduce the thickness of the drum and rotor and consequently its overall service life.
Others say that in spite of what the drum and rotor manufacturers say, new drums and rotors should always be turned to make sure they are true and flat. It is better for you and your customer to take a thin cut to make sure everything is right rather than risk a comeback because of noise or pedal pulsation.