Your best bet is to wait to wash until the quilt is completed and bound. To control any tendencies of the different fabrics to shrink differently, I strongly recommend doing a moderate amount of quilting -- no matter what batting you use. Once quilted, the batting controls shrinkage -- which means that even if the unwashed muslin wants to shrink dramatically, it will not be able to. This does not work for a tied quilt or if the quilting lines are too far apart -- say 10". If your lines are 4" apart or so, you should be fine.