I would prewash the fabric, I fill up the bathtub with medium warm water and retayne, and lay the fabric in the tub, follow the directions on the retayne, then swish around, drain, and fill again to rinse. Then I squeeze roll the fabrics, put them in old towels and roll them. I air dry. This is just what I do if I am concerned about bleeding. If the colors are not dark, I don't prewash.
I do not prewash a finished quilt usually, as I like the feel and look of a new quilt when I gift them. I add a package of color catchers with the gift.
I do not machine wash fabric anymore, as I "despise" the strings that get wrapped around everything. It is just a quirk with me. Each of us quilting do what we feel comfortable with, and with color catchers on the market, and retayne maybe prewashing is a thing of the past. The quilts and comforters and sheets in the store are not prewashed - if they were they would looke "used", something that I do not want my "gifted" quilts to look like.