I don't prewash. I like the body of the fabric that isn't. There are a few exceptions to this. I do wash certain red's, blacks and overdyed purples and blues. When I get the quilt finished I put it in the washer with a Shout Color Catcher and that pulls all of the dye out of the water and on to it. With your prewashed fabric I would use a spray sizing to give it some body when you iron it.
Debbie