I usually let it run big sims some special day, when I may leave the computer and not have time to play games, so I can check it once in a while.
I haven't tried letting simulate for a whole night, because I don't want to waste power, but if I could somehow determine for how long a sim would be "interesting" after starting it, I might do that one day. I don't want to make a sim of a planetary ring with massive bodies in it, if I risk that all the bodies merge into one moon like 50 minutes after I leave, and the com just wastes power on simulating a moon orbiting a planet for 8 hours.
At other times I just do random things in the sim if I have a few minutes before I have to go or before I go to bed, or if I don't know what to do.
But no, I minimize the program or turn off the screen when I'm not watching the sim. I don't know what determines if it renders the sim or not, maybe it always renders it...