Most shops that sipe just run the cutters around the tire, letting the sipes fall where they may (sometimes in the middle of the tread block where they might help, sometimes on the corner or edge of the block which does nothing). We feel that if siping helps the tire, we let the tire manufacturer put that in as part of the tread design from the beginning. Unless the ship can target the center of each tread block for a sipe, its just too erratic to be really helpful.