Q: Update 10.8.5?

Look at the specs and decide for yourself: OS X Mountain Lion Update v10.8.5 (Combo)

I always apply system updates shortly after they are released if for no other reason than once Apple publishes the details of the security fixes they contain, systems that are not updated have confirmed vulnerabilities some hacker might try to exploit.

Keeping the OS updated is arguably the most important single way users can protect themselves from

malicious software attacks. OS X is not invulnerable to such things; the best we can hope for is that Apple patches the discovered vulnerabilities before they can be exploited. When lots of users hesitate to apply those patches they indirectly encourage the malware writers to keep developing & deploying their attacks because there is a greater chance they will succeed on the un-patched systems.

That said, I always make sure I can "roll back" to the previous version if the new one breaks something important. The best way to do this is to clone the startup drive before applying the update; keeping an up-to-date Time Machine backup also works, but is more time consuming.