Never! Well, okay, there may be some extenuating circumstances.
According to WISC Madison, "bridal law" would have it so that all guests have up to a year to send their gifts to the happy couple, especially if you're one of those unfortunate few who can't make it out to the reception. It may even be a good idea to deliver presents a month before or after the wedding, if only to avoid the risk of anything getting lost or stolen at the actual event. Consider this yet another way to ease some of the burden off of the undoubtedly harried couple.
Of course, how you deliver it will have an effect on your presentation as well. Beyond all the wrapping paper, bows and trim, there's an undeniable element of interpersonal flavor that goes in to handing off a present in person. Will something be irrevocably lost if you send it by mail? Consider dropping it off at their home yourself if you don't live too far and would rather do it off the wedding clock.
For those of you who think a little extra attention to detail is all it will take, we encourage you to go to town with the packaging and presentation. Small extras will look oh-so-elegant when tucked into sheer organza bags, and you can always top off a delectable prezzie with a midori organdy ribbon.