I once ordered several bolts of the same type of backing fabric that Marshall's carried, for our store... and I would never order them again. They were quite rough to the touch, but worst of all is that they were wrapped so crookedly on the bolt, we had to give an extra foot on every cut, just so that our customers would wind up with a straight piece the size they needed. Bottom line... order extra.
The fabrics were inexpensive, but... depending on how you sleep, it may be the backing of the quilt that will be pulled up over you and against your skin as you sleep, so IMHO I'd rather have something that is nice and smooth.