Well, you don’t have to do anything. But look at the big picture. Even if you don’t know the newlyweds very well, they thought enough of you to invite you to witness their big day. Would it hurt you to send something—a modest gift or a card—to acknowledge it? Of course not. Would it possibly make the newlyweds just a tiny bit happier? Of course. So just do it. When in doubt, share the love.