If you cut just the width of piece plus track, you'll sometimes find thin uncut material on the bottom of the board, depending on thickness variations, pressure while sawing and the bending of the wood. It's also good to overshoot the depth a couple of milimeters as you don't have to go as far down the track at the end. On more than one occaison I've deplunged the saw thinking I was at the end of the cut only to find the lower corner still uncut.