Up to the user really

There are two schools of thought here. The first (the one I'm in) being that meat time between failures will be longer the less time the computer is on (shut it off, already) and the second being that a computer is happiest at operating temp. This is kind of like how the engine in your car wears most during the initial start up (when there's no oil pressure, and the temp is down) or in other words, leave it on it's happy that way, and won't go through constant heating and cooling.