You can buy quilt backings up to 120" wide for very reasonable prices - so you don't have to piece the back unless you just want to. :)
I buy most of my wide backings from Judi:
http://stores.ebay.com/QUILT-UNIQUE-FABRICS-AND-GIFTS - she's got about 120 different ones from $8-12/yd. Very reliable seller and FAST! :)
When you do piece a back, you can sew two widths together side by side, so that one seam runs down the middle. (or across the middle, in the case of horizontal piecing)
Or you can place the two widths together and sew along both selvege edges to make a tube, then cut one layer of fabric right in the middle of the two seams, along the entire length. This gives you one wide piece in the center and a smaller piece on each side.
Some people piece leftovers from the front into a strip for the back - I haven't tried that because I'm not real great at quilting normal amounts of fabric, yet, much less doubled amounts. :) But, it can have a very pretty effect and it thins down the scrap bag.