I always reboot systems after updates. Because that way I know that they can still be successfully booted! I figure I’d rather find out right away, instead of a year after the update, if a patch broke my init system.
But a well designed system can be rebooted without affecting service. Think of DNS, for example - even though it runs with 24x365 availability, I can reboot DNS nodes in the middle of the day whenever I want… as long as I don’t reboot them all at once.
