I've really been enjoying adding in "orphan" blocks or leftover pieces of fabrics into my backs for the last several years. I usually work with scrap quilts and usually make more blocks than I use on the front, so it's a good use of them. Or sometimes I have a themed fabric in the stash that just didn't play nicely with the others on the front but not big enough for the back. I should take a picture of the back of one of my cat quilts that used a number of large pieces...
Recently I finished a baby quilt that was just a couple inches wider than standard width fabric. I had a really nice jungle print that was long enough and went well with the fabrics on the top, and simply put black strips (it had a back background) for the extra couple of inches on either side. I had to be rather careful loading it on the long arm so that the sides stayed even.
Like others who have already posted, I usually use at least a 0.5" seam with fairly large pieces of fabric and always press them open. But then I press all my seams open. I've used multiple pieced backs on my little old sewing machine and on a long arm. Although I have left them intact if they are short and soft, if you have enough width you should cut off the selvedge for the usual reasons (thickness, twisting, etc.).
I've been using pieced backs for years, it was that or sheets before the wide fabrics became readily available with no problems as well. You do want to avoid having the seam in the middle if you can, but sometimes that happens depending on the width of the fabric. Still, if you can instead of having a left and right piece, put a full piece down the middle and half pieces on either side going the long way. I've also used three widths of fabric going horizontally for king sized quilts.