I made two diamond log cabin tablecloths years ago using my serger. One had no backing and one had a bleached muslin backing. I much preferred the unbacked one because the pieced top and the muslin had different shrinkage and kept it from being flat, even with ironing. I think flannel would have an even greater shrinkage problem. I use pieced tablecloths without batting or backing, I just hem the edges. The serger makes the back very neat, no raveling. If you don't have a serger, then I would use a cotton backing similar to the pieced top, but no batting. By the way , the diamond log cabin pattern makes a round tablecloth.