I turn it off when I'm done with it. I will have my second computer up and running soon with a couple servers on it later. But I dont keep it on 24/7 or 24 hours at that.
I leave mine on 24/7 usually.. but i don't worry about burn in with 2 17" LCD monitors. I turn the monitors off anyways to save what little energy they use. (force of habit)
As long as mine wants to keep running, it stays on. If I feel it getting a bit warm, I'll shut it down. I manually turn my monitor off whenever I leave for an extended amount of time.