Late to the thread, but a topic that has recently come up among my quilter friends. I have been quilting for nearly 40 years and bought into the idea that fabric needed to be washed before use initially. I detest ironing (of any kind) with a passion, so I hated this part. I decided to skip this step with my second full sized bed quilt and have never washed clean fabric (I do wash something that looks like dirt got on it to make certain it will wash out before use) in the over 70 quilts I have made since then. I have never had any fabric bleed onto another fabric, and for a while I was doing intricate Baltimore bride applique quilts on white backgrounds. Until very recently, I hand quilted all of my quilts, so there were many hours invested in each one. If I had noticed a problem, I would have prewashed, but go by the maxim "if it ain't broke, don't fix it."
Where I live (southern CA), water is a precious resource and I don't see the point of wasting it washing fabric that will get washed following completion of the quilt. I will say that I tend to wash in cold water with a cold rinse and either dry out in the sunshine or a hot dryer. If I use polyester batting, I get almost no shrinkage from the finished quilt to the washed one. Using unwashed cotton batting (warm and natural for the most part), I get between 3-4% shrinkage. I like the way the quilting looks with the shrinkage, so do not worry about prewashing cotton batting either.