I have a friend who uses a quality muslin for the backs of all her quilts, especially if she hand quilts them. I use whatever is on sale or I have on hand. I have even used up orphan blocks and left over fabrics, pieced them together and used that for the backs then I have used two yards of one and two yards of another if that is what I had on hand. For my extra special quilts, that are definitely themed, I find a fabric (generally on sale) that is real close to the front. I have also made sure to use up what fabric is left from the front for the back and added into with coordinated pieces. I think it is just a matter of preference you have. If you have two pieces that work, then just do the meenie minie moe thing and go for it.