We recently added support for CentOS 6.5 to PuppetAgain. We're upgrading puppet masters to this version first. The timing isn't terribly critical, but it's probably best to run 6.5 as that's what moco, relabs, qa, and servo are running.
I've upgraded all of those puppet masters to 6.5 already.
The upgrade is pretty straightforward: download
http://hg.mozilla.org/build/puppet/file/tip/setup/puppetagain-centos-upgrade and run
sh puppetagain-centos-upgrade 6.5
This will finish by running puppet and giving you some advice. You'll need to reboot the puppetmaster afterward.
The only problem we've encountered so far was a Ganeti VM that didn't come back up, but that was probably unrelated to the OS upgrade.
Dustin