My DH and I rent the house that we currently live in and unfortunately we have an ooooooold septic tank. We try to limit the amount of "washing" we do . . . so I can't really wash fabric any time I buy new. So what I do is fill my bathtub up with water and a tad bit of my laundry detergent. Then I soak one color at a time to get any excess whatever off the fabrics. I just ring out the fabric and put them in the dryer all together :-)
I have heard that if you do not prewash your fabrics and you wait to wash your quilt AFTER it is complete that sometimes you will get an effect of a rather old quilt . . been a long day so my brain doesn't really want to work LOL