Having been raised on a farm here is my experience.
Most gentleman farmers I know put there tractors in the garage, along with implements, or find some solution to keep them protected. Most farmers I know either let there tractors sit out, or under an overhang. I have never seen any farm implement sitting anywhere but on the ground, outside in the elements.
Sun is probably your greatest enemy, and its effects are cracked hoses and tires. I doubt rain or snow have much effect on any of your implements, other than maybe surface rust.
A tarp probably does the same level damage as not using one. Tarps hold moisture in, but keep the sun out. For me in the PNW water is the evil, not so much the sun. I do not have weather checking issues, but rust comes quickly.
I do not worry about water getting into a gear case (lets say on top of a mower deck). If water is getting in I have a greater issue of oil getting out. I do, however (or at least used to) block up my brush hog so the rain did not sit on the deck, but pour off. My current tractor the mower deck is a different design and water does not pool.
We spend anywhere from a few thousand to as much as a 100K to protect very thick pieces of steel. The math does not work out. A rusty box blade will sell for nearly the same as a freshly painted one at the end of the day. and how often are you selling your box blade.
I would, however, shed a tractor if I could. The newer tractors have a lot of electronics which I am still not convinced are as well protected on a smaller tractor as they are on a larger tractor. And hoses are not cheap nor are tires.
But, in practicing what I preach. In running a farm, the newer tractors were in the shop over the winter, the older along the side of the barn. The implements sat outside, but the area stored was either gravel or mowed grass. All machines received full lube and service before the 1st snow fall. When I bought my small tractor the first four years the machine and its implements sat in the garage, my work truck out in the yard. It wasn't until and old buddy from my Wyoming childhood came by and made fun of me did I realize I was being far too precious on heavy duty equipment.
But, your mileage may vary.
Carl