I'll turn it off at night, that's it. I never really considered constantly leaving it on but I'm not sure why I'd want to at night. I would never get a StreetPass anyway... then again considering my lack of success with StreetPass, I probably would
Turn it off pretty often, usually as I'm going to sleep. Although ever since I got the Mario alarm clock (yes, I bought it--using my 3DS as an alarm and waking up to Mario music was worth the 2 bucks for me), it's been on for multiple days at a time.