At its best, a birthday, graduation, or wedding gift is a token by which the recipient — usually someone you love, or at least know and like a lot — will remember you fondly. But what about when invitations arrive from very casual acquaintances? At these times, even the most generous among us have asked ourselves, "Do I really, really need to buy a gift?"

Happily, there's a simple rule that seems to be universally accepted by etiquette pros: "If you go to the event, you're obligated to bring a gift. If you don't go, you can nicely send regrets and there's an end to it," says Peter Post, a director of the Emily Post Institute and the author of Essential Manners for Couples (HarperCollins, $22, out in November).

As with all good rules, there are exceptions. With a wedding invitation, even if you decline, you should still send a gift. "Typically, you're not invited to a wedding unless you know the people well," says Post, so you should want to give something; not doing so would make too obvious a statement. This does not include those occasions, he adds, when "you get an invitation and you think, What on earth is this about? I hardly know this person." You can't help thinking someone's just fishing for gifts, and you don't want to bite. Post's proclamation: There's no reason to feel a bit of guilt about not sending a present to a virtual stranger.

Then again, what do you do if you're invited to your best friend's daughter's three bridal showers this summer? Even if you go to them all, says Post, "there's absolutely no need to bring a gift to more than one. In fact, we suggest that brides don't put people in this position." If you do multi-fete, don't feel awkward; your friend and her daughter will know you've given your gift. But if the courage of your convictions fails you, you could bring a small token — or send regrets.