Greasing the old hitch ball is debated about as much as towing with the LP on...
I have heard everything from nothing, to grease, to wax paper! Some say grease is good at first, but then is contaminated by dirt/dust. It is then bad for friction... I agree, old and dirty grease is just about as bad as no grease.
I use grease but don't go crazy with it. Before hitching I'll put a small "blob" of grease on the ball and hitch up. I clean it out with paper towels at the end of the camping season (winterizing) and apply some fresh grease.
2005 Ford Excursion V10 4x4 4.30's
2011 Salem 26TBUD