Sheets work very well for backings. I know that there are some quilters who will disagree but I have often used them. They machine quilt easily, wash and wear well and come in a multitude of colours. I look for the cotton rich sheets in King size so that I have extra to make matching pillowcases. Quilting fabric is at least $15 a metre in Canada and it's hard to justify the extra cost when a sheet can be purchased for about $10 if bought on sale. I buy them at Len's Mills, Giant Tiger (small department store) or Sears and they all sometimes have seconds available (can't often tell why they are a second) and that's when I stock up. We use sheets all the time for the back of our Community Quilts stitched by our quilt guild.