I piece the backs with huge squares; whatever I can put together to make it fit. I like the back to complement the front, so do match fabrics and often use some of the same in the back as was used on the front. The only trouble I have run into piecing the back is centering the front if I have pieced a regular pattern, so I tend to make one-way stripe pieces or other such things that you don't notice if they're not all there.