Reverse DNS: Standard? Spam fighter?

drmartin66 writes: Last weekend I installed a new spam filter server for a client and enable connection rejection if the sending server did not have a Reverse DNS record. Since then I have had a number of emails rejected from regulator bodies that do not have a Reverse DNS record and are refusing to have one created for their email server. What is the Slashdot communities opinion regarding Reverse DNS records, are they (should they) a standard and required or are they useless for spam fighting.