Unless I'm right in front of the machine, or, plan to be back to the machine in a short period of time, the computers stay off. Waste of electricity, and always the possibility of a machine or electrical failure to kill it.
And, I'd make a question of who does leave theirs on all the time.
How often have you opened it up to dust it? You HAVE TO do this reasonably often; a 1/16 layer of dust could cause an important part of the machine (cpu, power supply) to overheat and fail.
And you'd be surpised how much dust the machine collects.