I piece my backs all the time. I have never used wide backing fabric. Keep meaning to try it. Sometimes I press the seams open but mostly I press to the side. I treat it a lot like the piecing of the top. When I put in blocks, I usually but them off-center so I don't have to worry too much about center lines. But I have done them kind of "medallion-style" too. A block or piece of fabric in the middle, then other fabric around it. I don't worry about the quilting. After it is washed, it all seems to work out.