I buy wide backs, mostly from Keepsake, when they are on sale. I usually have one in my stash that goes well with any given top. I also piece backs from yardage in my stash. Usually I have enough scraps from the top to use for the binding. Since I quilt on a longarm, and need an extra 4" of backing all around, that is usually plenty for binding if not enough was left from the top, and if it matches the top well enough.
However all the above depends on having a good stash. If you're one of the wise people who buy just what is needed for a particular quilt, then I would recommend buying fabric for the binding when you buy the fabrics for the top. If it's a big quilt, consider buying a wide back on sale, or try to use up scraps from other quilts.