I wouldn't wash kit fabrics. As long as you quilt reasonably close together, the fabrics won't shrink that much once they are in the quilt.
I took a class from Harriet Hargrave once, and she brought a *flannel* quilt to prove this. She had not prewashed the flannel, but had machine quilted normally. The quilt was fine. Her point was that quilting stabilizes the quilt sandwich, preventing unwashed fabrics from shrinking overly much.
Now, a tied quilt would be a completely different story. There would be nothing to stop the fabric from shrinking.