Dealers leave tractors outside for months. I'm sure manufacturers leave them outside until they are shipped (plastic on the seats, perhaps).
It would really depend upon the duration. If a few months, I'd throw a tarp (not plastic...doesn't breath and more likely for condensation). This would be more to reduce fading from the sun. More then a few months, I'd consider a temporary structure of some sort.
BTW, make sure your fuel tank is fuel with some kind of fuel stabilzer in the fuel system.
As far as the implements...non-mechanical ones (box blade, rear blade), I'd just leave them set. RFM's I'd put under some kind of cover, if possible. Again, implements set outside at a dealer for months (in the old Tractor Supply in York, PA, KK implements sat in the shopping center parking lot) with no ill effects.
I'll leave my tractor outside for a few days...up to a week, with no cover. I do leave it outside and covered with a tarp if a snow storm is in the forecast. The tarp is more for the operator's (me) benefit then the tractor. In my case, there is little clearance between the ROPS and garage door. A few inches of hard pack snow or ice on the snow can give me a problem backing out of the garage.
