BIND will round robin the MX records, how mail agents deal with multiple MX records of the same priority is less of a known factor (to me at least). Actually I'd be interested to hear from people who try both configurations for say a month at a time to see what influence each configuration has if any.

RFC 974 reads: D.EXAMPLE.ORG IN MX 0 D.EXAMPLE.ORG D.EXAMPLE.ORG IN MX 0 C.EXAMPLE.ORG D.EXAMPLE.ORG IN WKS 10.0.0.4 TCP SMTP In the third example, consider a mailer on A.EXAMPLE.ORG trying to deliver a message to D.EXAMPLE.ORG. In this case there are only two MX RRs, both with the same preference value. Either MX will accept messages for D.EXAMPLE.ORG. The mailer should try one MX first (which one is up to the mailer, though D.EXAMPLE.ORG seems most reasonable), and if that delivery fails should try the other MX (e.g. C.EXAMPLE.ORG).