While spammers certainly don't worry about conforming with RFCs, it should be noted that rejecting email based on the DNS is a specific violation of the relevant RFC, RFC-5321, section 4.1.4, which states:

An SMTP server MAY verify that the domain name argument in the EHLO command actually corresponds to the IP address of the client. However, if the verification fails, the server MUST NOT refuse to accept a message on that basis.

That said, it is common practice to do this anyway. The reality is that you will not have reliable email delivery unless the hostname your server issues in the EHLO command resolves to the servers IP address and the IP address resolves back to that same name.