When if doubt, I prewash fabrics, and yes I have used the muslin from JoAnn's. Just remember the origins of quilting started with scraps of anything people could get their hands on...mostly old clothes. No one had quilt shops they went to and spent big money on fabrics. I know there is a ton of opinions on the subject, but for myself, I am not a "show" quilter. So for my projects a variety of fabrics are just fine with me. My very first quilt still hasn't disintegrated, been eaten by bugs when I used starch, or any of the other horrors we've heard. And that for me is rather unsettling, since it was pretty awful, and I would have prefered to see it die in a slow death!!! hahahahahaha