It's something you hardly ever do. Even if there's been some kind of installation/update that requires a reboot, it tends to be able to trigger that itself (which can cause other pain, but that's for another day).
There are some people who like to shut down their machines, though. I think they haven't forgotten the days when a good reboot once a day was necessary to clear out cobwebs.
That's what I figured. I reboot my Mac when updates require it, and every once in a while if something weird is going on I might reboot it, but by and large my computer has uptime's in the weeks.
On OS X it's somewhat rare for an update for require a reboot. Often they don't actually reboot the machine but just log you out, perform the update, and then log you back in.
While these updates feel like a reboot since you lose your desktop environment state, you can tell the difference if you're paying close enough attention.
There are some people who like to shut down their machines, though. I think they haven't forgotten the days when a good reboot once a day was necessary to clear out cobwebs.