Rules? What Rules?
I'd say 70% of the time my binding matches, 15% of the time I use one of the other fabrics because I'm running short and the other 15% of the time I choose to have a contrasting binding. The back is whatever I can get or have that looks good with the binding and that's the back! I like to get extra yardage and then use the large scraps in no particular pattern for a pieced backing. It's like having two quilts.