doesnt hurt too much....rear tires spin....front brakes are holding the car still...rear brakes do little to nothing....spinning tires means little stress on anything....thats why traction kills cars...not spinning
i really would recomend spinning the tires all the time tho, at the track all you really need to do it spin em a bit to slean em off unless you have slicks. Plus it's a ****** getting rubber off the bottom of ur bumper