Guess I'll be the odd man out here lol, been in and around the construction business for 15 years. Never have I greased a ball or hitch or seen anyone else doing it that I can remember. Have hitches,balls, and trailer couplers 15+ years (and 100k+ miles) old that still work fine thats never had grease.
...but, if it works for you thats all that matters.
I do use a teflon plate on my fiver.
I wouldn't really be against greasing any hitch,ball, or coupler, but I can do without the mess of it. Its worked fine for me thus far so I see no need to change now.