Backgammon stack rule
Stacking pieces on the same point is not necessary unless it would otherwise be unclear where those pieces are meant to be. There are no rules about how you place pieces on a point; it is only common sense that mandates clarity on exactly which point a piece occupies.

You could stack every subsequent piece on the first one on the same point, if you really felt the need. However, this is strongly discouraged, because it may disguise the number of pieces on that point, especially if the point has only two pieces.

You gain nothing from stacking them before you need to, and risk an accusation of poor gamesmanship. Why bother?