When I first started quilting I thought all backs had to be solid color (think I based that on what my grandmother did). So I bought a lot of Kona cotton solids. Didn't take me long to realize I like to coordinate the back to the front. So my suggestion would be put on the back what you think would work the best (muslin, print, or whatever). Then only purchase backings as you complete your first few quilts and I think you'll see your opinion and taste change as you grow more confident with your fabric choices, colors, and patterns. The last few months I've actually had the backing picked out before I've gotten all the fabrics for the front. It's always fun for me to have the back be kind of a surprise for someone when they look and discover there's more than just the front to a quilt.