As bad as people are w/ RSVPing, "regrets only" makes it even worse. If you don't really care about the #'s, then sure, do regrets only. But if you want some concept of how many people are coming - you need to get RSVPs and you need to follow up w/ those you don't hear from.
I went to a "regrets only" shower this summer. They were expecting 60 people. I think about 20 or 25 came. They had so much food left over. All because they based it on those who only said "no, we're not coming".