Well I guess you can treat your balls as nicely as you want to. I personaly dont lubricate mine, although it may be something to try. It might help a little, but I think that you can do just fine with leaving them dry. As far as replacing, you should get new balls when your old ones are worn out, or beaten up.
I think it depends more on the size of your balls. If they are the big balls (2 5/16) they need more grease. Don't bother if your balls are small (1 7/8). another trick is to powder them when it is hot, this will make them not stick to the tongue.
Personally, I like mine greased as it holds a shine better when buffed. All kidding aside, I use a teflon coating on my trailer ball. Does it work? Heck I don't know. But the teflon was suggested by the dealer who sold me my trailer at Expo 2000.
Ok gentelmen, the truckers allways grease the hitch plate because they dont know if there going to get a dry trailer.
The same rings true on our smaller scale. Friction is the main reason to grease anything, from your mower to the truck ball. Bearing grease or any multi purpose lube dabbed on top of the ball works fine. Say didn't AC/DC have a song about big balls?