I use heat press batting tape. As an added secure measure, and since I don't trust that it will stay ironed onto the batting, I turn over the batting and ladder stitch the seams by hand. I won't put the batting under the needle without fabric on each side. When ironing (press, don't actually move your iron back and forth) you are butting the seams up. When the batting is turned over, the seams are perfectly set against each other, no layering, no bunching. Done this to many many quilts and after washing-no problems what so ever. I buy the large rolls of batting, so when I make the quilts and they are larger than the width of the roll of batting, I place the straight edge of the batting one third of the way in from one edge, and then butt up the other piece. You want to trim the raw edges around the quilt anyway, so why use up the good straight edge you already have. I never have a seam going down the center.