muslin is one of my all time favorite materials for quilting. i have backed many quilts with it and used it for background also. it surely needs to be washed first, shrinks a lot.
it also comes in many different widths. be aware that it may be wider or narrower than we are generally accoustomed to. but after quilting it is a beauty. i use it to achieve an antique look on my scrappy quilts.
hope this helps.