I do not prewash.
1. If you like the wrinkled up anique looking quilts as I do, don't prewash fabrics and use Warm and Natural batting.
2. Wash in hot water with dye catcher.
3. Dry in dryer on reg cycle.
This will shrink the fabric and give it that lovely old look that I particularly like.
So, it really depends on the look you want.