I have never applied grease to my trailer hook. As a result, it looks like this :
I have been told that I should not grease it (something to do with the friction being important for... something?).
I can see that many people do grease theirs, and then cover it with a cap, or tennis ball.
A Google search reveals a wide variety of fairly strong opinions on this apparently innocuous subject. What is the correct treatment, and why?