When you piece a backing, just make sure to cut off the selvage edges, as that weave is so much tighter than the rest of the fabric, and will make your backing pucker. Press the seam open. As for centering, it is very difficult to get a backing seam to line up absolutely perfectly, due to the give of the fabric, the fact that it is mounted on rollers, the height/weight of batting, quilting from left to right, right to left, etc, or that the quilt itsself may not be pressed perfectly flat or lie perfectly square. And for certain, do NOT put a border on your backing and expect your longarm quilter to line it up perfectly.
I recommend that if you piece your backing, (which adds value to the quilt, according to most quilt appraisors), piece it somewhere in the middle, but not exactly through the center. Add a row of leftover blocks for interest, or a stripe of contrasting fabric. Have the seams lie somewhere that there are not a lot of seams or joints on the front of the quilt. And then, sometimes, just piece it and hope for the best! :)