An alternative is to watch bargain stores for cotton sheets. Sometimes you can find them for less than yardage prices. I have a couple that I found still new in the packages at old estate sales and thrift stores. Wash them very well, iron and they make wonderful backing with a feel that modern fabrics just can't compare to (IMHO).

If I'm using regular yardage, I have always placed a full width in the middle (selvages removed) with half a width added to each side of that, for the whole length of the quilt. This places the seams to the sides of the quilt instead of straight down the middle. They seem to present better that way when completed and they also seem to wear better over time.