You simply can choose NOT to restart when the computer asks you, and all the updates will be installed at next boot. That's what I usually do if I'm doing something important when the update message flashes up.
About switching off computers: I switch off mine at night because it's in the same space where I sleep and the noise would annoy me, otherwise during the day is usually on since I'm working on it and it allows me to receive in real time e-mails or other communications. When I'm away for more than some hours, it's obviously switched off.
Leaving computers on is generally better than switching them on and off 10 times a day, but of course it's pointless to leave them on 24/7 unless you're running some application like p2p that actually "uses" your computer also when you're not on it.
Expecially for PCs, however, it's "healthy" to reboot them sometimes, while I heard that Macs can go for years "suspending" the system instead of properly switching it off.
