I piece batting all the time and I use warm and natural/white. I usually put the two pieces with a little overlap and cut in the the overlap area so I have matching edges. You lose a little batting this way but not much and the edges then align perfectly. I usually whip stitch the edges together but have zig zaged them too. I would think because the batting is already sandwiched in your quilt it would be easier to lay it on a flat surface and just whip stitch it together by hand. Since the area will be quilted and also protected inside the top and back of the quilt there should be no problem with having it pieced. My items that I piece batting (quilts, pot holders, you name it) are washed and used a lot and I've never had a problem.