I'd say that if you don't need to update FreeNAS and it's in a secure location and not exposed to the internet, then don't upgrade unless there is a solid reason to do so. But in a business sometimes that is not an option and you could be forced to take all the security updates. As others have said, test out an update before applying it to your servers. With FreeNAS this is a very simple thing to do because you could update a single server and test it out and if you don't like it, just roll it back. NEVER upgrade your pool unless you have run the new version for a few months and you know that you will never roll back to a previous version. I prefer to not upgrade my pool at all but that is me.
