People want personalization. We want to make sure people receive it, read it, and respond to it. People like the ease of responding through mail, but also e-mail, fax, and voice mail. Make responding easy. Don't use labels or bulk mail.
Bulk mail cheapens the event.
Then, there are those who don't respond, but show up, or who accept, but don't show. To protect themselves, many groups now add a surcharge for registrations at the door and hold members accountable for reservations they make. If you're going to spend the time and money for an event, make sure your invitation conveys the right message. In our business casual world, gift giving can become gift slinging. If you're going to the trouble, go to some trouble.