I do not prewash my fabrics. I do wash all quilts before they leave my house. I want to make sure they are clean, free of dog hair and since I use a basting spray, I want to make sure that is gone as well.
I have a front loader washer. I just run it on a casual cycle with low spin, a little detergent (I use Amway stuff) and a color catcher. I then toss in the dryer on a lower temperature.