I agree with Dolphyngyrl about the prewashing. (I would do the same with muslin for embroidery, by the way!) Once fabric is quilted, the batting takes over how much the fabric can shrink.
Harriet Hargrave brought a flannel quilt to class to demonstrate that even flannel does not need prewashing (for shrinkage) as long as it is quilted reasonably well to the batting. She measured the quilt before and after washing; it shrank exactly the amount that the batting predicted. (Batting was Hobbs 80/20.)
Ever since that class I have never bothered to prewash. If I suspect a fabric might bleed, I test a small piece and wash only if it shows a lot of bleeding. I always do the first wash of a quilt with Synthrapol in the largest front-loading washing machine at the laundromat. This ensures that any small bleeds will not settle into other fabrics.
I can understand prewashing fabric if one is sensitive to the chemicals in fabric, or if the fabric is really dirty. Otherwise, I just don't see the need to expend all that time and effort on something that is really unnecessary.