Sometimes do the top button, always do the middle one, and never the bottom one
Most Helpful Guy
Single breasted, that is pretty much right. Never do the bottom button as it actually distorts the fabric, plus it makes sitting awkward. The top button should be done or left undone depending on what looks best. Doing it may seem to scrunch up the top, where as leaving it undone may cause the suit to flare open more easily if you are moving around and twisting your torso.
However, you may actually be better off with a two button suit or a double breasted suit, especially if you have a shorter, wider torso. With the two button you should only fasten the top. As for the double breasted, the only rule (that I know of) is that it should stay buttoned while you wear it. The exact configurations of what buttons you choose depends simply on what looks best.
Hope that helped.0