Sometimes, not using any grease will result in the ball turning and loosening up - even with it being properly torqued. This happened to me many years ago when we first started towing. The steel on steel friction was enough to turn the ball and it was bad enough to wallow out the hole the ball mounts into. There is a terrific amount of pressure on the ball and coupler when the WD bars are drawn up.
Ever since that happened I have been a strong advocate of greasing the trailer ball every time you go for a tow. Doesn't have to be globbed on just a thin even coat. I always wipe it off in between trips and re-grease before the next one. Just takes a minute and you don't get grease all over you pant leg when walking around it. Biggest benefit is that the ball wear and coupler wear is almost nil, and has never loosened up again in almost 40 years of towing.
Barney
