Common sense applies.

The heatup/cooldown cycle can take its toll on electronic components and connections as well, so if you go away from your keyboard temporarily during the day and are coming back to it again, you should likely leave it ON for those short periods. Of course, if there is the threat of a lightning storm, you should turn it off and unplug it from the AC mains anyway.

--Mac
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"Keep listening. Never become so self-important that you can't listen to other players. Live cleanly....Do right....You can improve as a player by improving as a person. It's a duty we owe to ourselves." --John Coltrane

"You don't know what you like, you like what you know. In order to know what you like, you have to know everything." --Branford Marsalis