Sometimes, yes, but only along certain dimensions. If your group performs rituals, they can't be rational because then they will be the same as other groups'. For example, the Jewish practice of eating flat bread on Passover is arbitrary [1], but it only works because it is arbitrary. [1] It's not entirely arbitrary if you believe the story of Passover, but that's a somewhat different point. Actually, it may be interesting to examine whether it's rational in that case—I can see arguments for both sides.