It would depend on your reason for prewashing. If you are doing it to remove the sizing, chemicals, soil & oils you would want to use detergent.... You can still do it by hand if you want. If you are doing it simply for shrinkage the dryer does as much as the water. If you are doing it because you have a 'bleeder',I would think you would again want detergent (and a lot of swishing) to make sure you get all of the running dye removed