Unless the fabric is really smelly, I don't prewash. After quilting I wash with my regular detergent on the regular/ normal cycle in my washer and throw in a few color catchers. So far, so good.
Smelly fabrics get pre-washed same way and thrown in dryer. If that doesn't get the smell out, they get run through a quick wash cycle with detergent and hung out to dry in the sun. For this method I fold it in half right sides together before hanging to prevent fading. I once had a fabric that was so stinky (smelled like cat pee, but was fresh from the printer so must have been the inks) that it took 3 days of hanging outside in direct sunlight to get rid of the smell. But it was a fabulous fabric at a very reduced price due to the smell, so worth going through the trouble.
Rob