I almost always piece my backs - just to add a bit of a design element. My quick favorite is to cut it in 4 pieces and sash them together like a giant block. But, if I am only piecing 2 long pieces, I tend to use one whole piece in the center and split the other to use along each side (think 1/2 a whole and 1/2). I just like the look of it rather than a seam down the center.