A lot of times if you prewash muslin, it will shrink and the weave will tighten up a lot. I don't normally prewash but, with a muslin like this, I would try washing a piece first to see how it turns out (before washing the whole piece). My bet is it will be fine after washing. (For a whole cloth quilt, though, I'd probably want to use a Moda or Kaufman Kona muslin. Be aware that not all Kona muslins are Kaufman Kona. Kaufman Kona is the good one.)