I do a mix of everything! I use wide quilt backing, but I tend to save that for quilts that cost more for the top fabric or are a bit more complicated. I also have a stash of fabric that I bought when it was on close-out at JoAnn's and there was enough left on the bolt to piece together for a back. For scrappy quilts, I'll piece together two or three different fabrics from my stash. Finally, I'll sometimes go to JoAnn's and find something special for a quilt.