I love muslin for lots of things. I always buy the more expensive muslin at JoAnn's and usually get the extra wide so I can use it for quilt backs if I want to. Look at the thread count and get the higher thread count. I use both the unbleached and bleached muslin, depending on what I'm doing.
Better grade muslin is wonderful for red work and other embroidery projects. If you do crayon fabric tinting, it is absolutely the best.
Watch out for both quilt police and fabric snobs! Both can wreak havoc if you pay too much attention to them.