I'm being paranoid here I'm sure. I'm at the end of an SQL Server 2012 install and this message appears. I know it says that I must restart the computer but I've had instances where installer automatically restarted the server when I didn't want it to. I cannot reboot this production server right now. Am I safe to click OK to complete the installation?
You can safely click OK and it won't reboot it. The instance will even be up, online and accessible without the reboot.
It's always good practice to reboot after a fresh installation or patches as when you try to perform another patch or installation down the road, the installer will more than likely fail of the 'Restart Required' portion of the checks.