I've been shutting it down, but I know some say it is more wear and tear on the computer shutting down and re-starting. Second question, if you leave it on do you leave your external drives running as well, or shut them down?
I've heard that the less you start/stop your hard drives, and the less you shut down/restart your machine, the better it is for it. Not sure how bad that impacts power consumption, or how much truth there is to it.... but I put my rig to sleep and only shut it down when I leave for more than a day.