You don't have to start your SITD in the center. Your quilt should be pinned well enough you can start anywhere. That being said, you can start at one edge and sew to the other. That way, the ends of your seam will be caught in the binding. If you do need to start in the center part of the quilt, take abut 1/4" of T-I-N-Y stitches - as small as you can get without being "0". If you've ever tried to pick out those kind of stitches you know they aren't coming out! That way you don't build up thread like you would if you backstitch.