Under 2003 it used to be, set the primary to itself and then set the secondary and tertiary to be two other DC's, usually the same for all others.
In our last couple ADRAP's the best practice has changed to set the primary and secondary DNS to be the same servers for everyone, and then set the tertiary DNS server to be the local DC's IP or 127.0.0.1
(we use 127.0.0.1, but that will cause dcdiag.exe to report failures when it does DNS tests, even though DNS functionally will work fine in this config) - so essentially the reverse of the 2003 best practices.
By no means is that a hard and fast rule, but if you have a large number of DC's in your environment (like we do) it does ensure DC SRV records are always updated on the same server which minimizes replication conflicts. Keeping localhost or the local IP as the third entry ensures DNS stays functional in the event network connectivity to the primary and secondary DC's is lost. (Source