You can do it either way, even leave a couple of knitted stitches at each edge. It really depends on what you want. If you do the decreases in a stitch or two, it can make seaming easier. If you do them at the edge, they won’t show as much, but sometimes the decreases add a bit to the look of the sweater.
This sweater has a heavy pattern - almost like a waffle pattern. Would it be better to do the decreases on the endmost stitches so it doesn’t break the pattern?