I will try to buy all the fabric for the top at once because I am a "matcher". I like everything to go together and when I am shopping, I am seriously shopping for the day until I get it done. So I generally will do the whole top at once. If the backing is calling then I will get is as well unless this is a top that I am randomly putting together and not sure about the total size. I recently did a "scrapbook" quilt for my son that included blocks of his favorite things like a camera applique, a car similar to what he drove, a dog like our lab, and a snowboarder. They were all different size blocks and needed filler blocks and strips between so I couldn't tell how much backing I would need until it was done.