I would not wash now not only to avoid the raveling of the seam allowances but also the quilting will stabilize the muslin to some degree and you won't get as much shrinkage with it quilted. It also depends on the quality of your muslin. Some higher quality muslin really don't shrink that much. The cheap gauzy ones do.
Someone correct me please if I'm wrong but I think the more quilting you have the less shrinkage you'll have because the fabric is secured in more places so you might want to consider that in your quilting design.