There are two possible problems here. One is that muslims come in different thread counts per square inch. If you get the really cheap flimsy fabric, it will be a disappointment on a quilt. I have no problem with the right kind of sheets on my longarm to quilt it -- but if you're going to have it quilted by a longarmer, contact them BEFORE you purchase the backing. They may NOT be willing to quilt it on a sheet. I just got a customer with 4 quilts --- she wanted sheets for her own reasons and she was turned down by a number of quilters. I see no problems with it and told her what type to get (no jersey sheets -- it stretches too much). Also, different longarmers require different amounts of fabric beyond the size of the top. Some say 4 inches extra on all sides, others say more.
If you piece the backing ----- use 1/2" seams and press them open ---- it lays flatter and nicer