I keep my Farmall and Kubota in a shed. My Kubota had sat outside before I got it. While it was in great shape excepting some paint fade, the tires were another story.
I got a nail in the side of one of the rear turfs. Had to throw them away even though they had tons of tread. The sidewalls were in terrible shape because of weather exposure.
Definitely worth keeping the tractor out of the weather, if for nothing else but those $250/ea tires...