Quoted below is my posting from May 07, 2010 re:108" wide cotton backing shrinkage: just a note of caution FYI.
I bought 108" wide fabric for king size quilt backing at JoAnn Fabrics. It's 100% cotton so preshrunk it in hot water wash & dryer. Well, its width went from 108" to 92" -- 16" shrinkage in width! Called the JoAnn store & clerk told me "It should've 'only' shrunk 8 inches"! If I'd been advised of that before I bought it, I'd have selected a poly/cotton blend instead. I'd bought an extra 3/4 yard, thank goodness, so ended up having to cut the excess length off & turn it crosswise grain and sew onto the sides to get enough width. Then I washed it again, in hot water, and dried it again to be sure it's done all the shrinking it's going to do, hopefully!