When I was much younger my balls were much looser and I was lucky to never have any problems from that. Nowadays no common preventatives can be relied upon to avoid problems and I worry for the younger-set.
I've read the concerns about loosening balls with interest and note that EVERYONE should avoid relying upon lock-washers and torque and/or grease to prevent loose balls. Trailer balls should be secured with locking devices such as cotters, pins, or by welding. Some hitches have the balls permanently machined to the hitch (my personal preference... I never want my balls to become separated from where they were intended to be permanently mounted.)
Tow & Stowâ„¢ Adjustable Ball Mounts | B & W Trailer Hitches
I am not a greaser. But I am an inspector and I look the equipment over regularly and replace/repair anything worn out.