Napa red stuff, Deere has a red spray on battery product (although it takes forever to dry, so it gets a little dust on it). Vaseline and grease insulate, but are messy. If you don't have any red spray stuff, I'd clean it, mow "battery commando", then clean and spray when you've got the right stuff.
The goal is in insulate the terminal by coating them. The other battery maintenance item is to keep the surface of the battery clean and dry. Thin films of water and dirt conduct current that can corrode the terminals, especially the plus one. If both terminals are getting corosion, it's acid from the battery and something is very wrong. The baking soda clean will also tell you if it's acid from the battery. If it's bubbles it's acid. If it's just the plus terminal that has corrosion, there is a small current "help" process going on. I also am _guessing_ that the problem is worse for tractors kept outside. They tend to get more condensation on them due to weather changes than tractors that are inside, especially in insulated spaces.
Next time I go to clean a battery, I'll take a conductance measurement first just for grins...
I end up wiping down my battery, especially around the + post, every 3 months or so. They get cleaned and re-coated with spray as needed.
I'm still thinking on the copper penny one... I'll have to go dig up my galvanic table of metals first.
Pete