If it's stored outside without a roof, I wouldn't leave them out. As others have mentioned, they don't seal as well when they're out and it'll shorten the life of the roof of the slide.
As far as leaving it plugged in is concerned.. I always worry less when it's unplugged due to the risk of surges, shorts, and possibly a fire.
I have covered, powered storage where I could leave my slides out and the camper powered up. I won't do either. It's just not worth it.