Those of you who do not grease the ball and using a WD hitch better be careful. The constant rotation of the coupler on the ball and the heavy metal on metal contact tends to often loosen the ball and it could eventually turn enough to drop the nut on the bottom, causing the ball to wiggle and egg out the hole.
If it is on a regular weight carrying hitch or drawbar, then there is a good possibility that the ball will come off the hitch with quite disastrous consequences. It has happened to many who thought it "couldn't happen to me".
Anytime you have heavy metal on metal contact you should normally use some type of lubrication. All hitch makers tell you to lubricate the ball except possibly at least one or two of those that don't rotate on the ball like the Andersen, although even it has up and down and side to side rocking motion on the ball.
Barney