What to do for a quilt back is up to you :D:D:D
Some use a solid piece of fabric, some piece them, some put leftover blocks in them... I would just keep in mind the quilting on the front, will also be the back sides quilting. Also in some spots you could have a seam in the front and back meeting and that is a lot of thickness with the batting, too.
Here are some pictures of different backs, maybe give you some inspiration :D:D:D
http://images.google.com/images?sour...ed=0CBMQsAQwAA