Your raggy quilt will still ( fray) well even with prewashed fabrics, which seems to be what everyone is concerned about... It is the clipping of the seams that creates the (chenille effect) I will admit I have made raggy quilts without prewashing my flannel a time or two which turned out just fine, but it does depend on the flannel being used. To be (safe) especially for your first one, prewash.... After you see how it all comes out and are comfortable then do some experimenting