When DH and I were planning our wedding (13 years ago), I was in charge of ordering invitations (well, and everything else. All dh had to do was show up). Anyway, I was telling my MIL how many invitations and RSVP cards I was getting and she told me not to bother with the RSVP cards for their side of the family, no one will bother sending them back. I don't know if it is maybe a southwestern thing or what. Everyone on my side of the family returned theirs (back east and a few up north). But with DH's family, we just guessed at who all would be there.

But I always try to RSVP. It's just rude not to!