On the waistcoat, I use all the buttons, as it looks better with my long torso. Even in "tall" sizes, when I was thin (27" kilt waist), they never were quite long enough to look right with the bottom button open. Now that I am fat (35" kilt waist), I am still needing to cover the vertical real estate, although one of my waistcoats is long enough to leave the bottom button open, the rest are not.
Slainte