I buy Roc Lon 403 and 404 ( bleached and Un bleached) its permanent press and pre- shrunk. The perm /press really does diminish the incredible amount of wrinking you get with some muslin. It does still have some shrinkage , but not like some of the others . I buy full bolts of each of these. I consider these to be staples in my fabric inventory.
All muslin is not created or performs the same. I bought some many years ago that once washed .. there was almost no way to get the wrinkles out, after that experience I learned a few things and found the ones I mentioned.
I do buy Kaufman muslin 108 wide for backings, its a very very nice quality .