By all means, don't hesitate to smear a little bit of grease on the hitch ball! I am a semi truck driver by trade and I can tell you right off the bat there are two reasons to put a coating of a lubricant on the ball.
First of all, the grease will help prevent corrosion. Plain and simple.
Secondly, if all roads were straight as an arrow to your destination, there would be no friction to speak of. The real world has roads winding left and right and up and down. Not to mention the road crown. All equating to friction and wear. Metal on metal, without lubricant, under those condidtions is not a good thing.
I am surprised the 5ver crowd hasn't said more. As a steel hauler, I am well aware of what damage weight can do to a vehicle without proper lubrication. Even cornering is much nicer when the friction is reduced at the pivot point with a little grease.
Let me put it this way...what can it hurt?
My advice is to bite the bullet and buy that $3.00 tub of a quality chassis grease, stick your favorite finger in it, and smear some on that ball! I'm not talking gobs of the stuff, but a liberal coating would be fine. I clean it all off in the Fall because the grease will attract sand and dirt. I then apply a new coating for Winter protection and storage. Ready to go come Spring. Maybe a little more for the towing season, then repeat. No worries.