Like most, I have done both. If I piece the back I use very large squares, generally just cutting up my fabric and sewing it back together so it covers the area I need it to cover (keep in mind the direction of the grain). My biggest disaster was when I used a contrast colour because I didn't have enough of my backing fabric (and live hours from a store). I staggered the contrast blocks, but when I trimmed off all the extra backing fabric, the outside blocks looked obviously smaller. Since then, if I use a contract, I would make it a stripe in the middle or somthing similar.