We had bought the backing frabic at the same time as the quilt front's becuase my sister lives in Wyoming and at the time I was living in California. I had an easy pattern in mind, and then changed it once I got home and realized I had enough fabric to do the quilt I really wanted to do for her. She wanted to be a part of the process, hence why we bought it all.
I do usually buy my backing fabric at the same time I buy the fabric for the front. I like to buy extra material just in case, and that way my stash stays nice and plump!