Slashfacepalm taking the strings off would leave the neck without tension. I believe that's something that should never happen as it will result in the neck being bowed.
Hm, right on.
This probably rings true for your set-neck guitars especially, but I'd have no problem leaving a Strat this way. But then again, I've never stored something that long.
I guess just tune them to D Standard and put some of that thin foam packaging paper between the frets and the strings. The frets do become marked after time if it just sits for a long time. That's one thing I do know.