I use the CentOS spamassassin package instead of the one in the Atomic repository, because these days CentOS (5 and 6) comes with SpamAssassin 3.3.1, while the Atomic package is still at version 3.2.5. Yum installs the Atomic package by default though, because its epoch version is higher. I don't know why this is the case, but I exclude spamassassin from Atomic in yum configurations for this reason and install the package from the CentOS repository.
Both the CentOS and the Atomic spamassassin packages contain /etc/cron.d/sa-update which updates SpamAssassin's rules daily.
The remark on the wiki about adding saupdates.openprotect.com is outdated, since that channel has been retired. http://saupdates.openprotect.com/
says: "OpenProtect' SpamAssassin sa-update channel is obsolete since SARE stopped updating their rulesets. Please stop using this channel." I've added two notes to the wiki page about this.