I used a ton of yardage the past two weeks preparing the backings and bindings for quilt tops. I've got a couple of rules for backings. They're simple rules - because I'm kind of simple.
First, I try to use any leftover fabric from the quilt top on the back of the quilt. It totally busts that fabric from my stash, and the backing is coordinated. Sometimes this involves getting real creative. LOL
This is the backing for one of the quilts made from a pattern I tested for Judy. It's called Carolina Stroll. These three fabrics were on the front of the quilt. I barely had enough, but it worked.
If there's just no way I can get a backing out of the leftover fabric, then I go to my stash and pull yardage. The leftover fabric from the top then goes into the scrap bin to be cut into strips. I do have a closet full of flat folds of five or more yards, as well as some nearly full bolts. Almost always I can find something there.
Occasionally I also buy yardage specifically for a quilt. For instance, if I sign on to a block of the month, then I go ahead and buy the yardage for the backing at that time. Kimberly at The Fat Quarter Shop is great about offering yardage cuts with BOMs and kits. I like having the backing in case I mess up doing the front. I know I've got some extra fabric that will match, and that fabric might not be available a year or two from when I ordered the BOM to completing it. It's a safety precaution for me. I learned this the hard way! LOL