I always piece my batting. To get a clean edge to butt up against each other I overlap the edges slightly and then cut through both layers. The edges now align perfectly. I whip stitch mine together by hand. It doesn't take that long when you lay it on a flat surface and for me is easier than trying to keep things together zig zaggin on the machine. You can buy tape for this purpose as well but it is pricey and I would rather spend my money on fabric.