I think prewashing is an invitation to disaster and making the kit unusable because of the raveling. Even if you wash and dry in a lingerie bag it will still ravel a lot and then it might shrink unevenly making it unusable whereas once quilting the batting will control the shrinkage. I would just use color catchers in the wash but I rarely prewash including flannel but I only buy flannel from connecting threads and it has minimal shrinkage.