I don't prewash unless I suspect a fabric might bleed, in which case I test it for bleeding first. Prewashing isn't necessary if, like me, you aren't allergic to the chemicals in the fabric, you like the old-fashionied crinkled look of a quilt, and your quilting lines are close together.
Quilting stabilizes fabrics and limits how much it shrinks. Harriet Hargrave showed us a quilt she made to prove this point. She used un-prewashed *flannel* (!) to make a quilt, machine-quilted it, then washed and dried it in machines. Aside from showing it to us (looked fabulous), she measured it before and after washing; because her machine quilting had stabilizes the fabric, it was virtually the same dimensions before and after washing.
I personally still pre-wash flannel, but that demo caused me to stop prewashing my other fabrics. Has saved me a ton of time!