If I see a perfect backing fabric while choosing fabrics for the top, I go ahead and buy it right away. For example, I'm making a baby quilt with pieced guitars and I saw barnyard animals playing guitars in a colorful print that matched the colors of the top and the theme of the top...had to have that print it was so cute.
I usually wait to choose binding fabric until the top is complete.
Often times, I will piece the backing and do scrappy binding from leftovers from the top of the quilt.
All in all, I don't have a consistent way of purchasing backing and binding fabric.