As with anything in the Quilting World...do what you want to do. Using a really good quality muslin fabric is important and easy to needle and also machine. Remember, it's your quilt and there are no rules only what you prefer your look to be.

I always use the extra wide muslin when I am creating a antique/vintage style quilt as I believe it is in keeping with that style of quilt. I have never found them to look dirty. With this particular style of quilt I do want it to look old/vintage.

That said on the other style quilts I use the wide backs as I prefer non pieced backs. That way my quilts whether using muslin or wide backs all have the feeling of whole cloth on my backs. Before the wide backs came out I never really liked the look of pieced backs and felt my quilting was not accentuated as much as I desired.

I ordered some Moda Muslin that is 108 wide. I've never worked with muslin before so this will be fun!

I'd use a good quality muslin and prewash it.