In my view, the 10% rule is only a guide, but also the maximum mass that should be placed on the tow ball. It is somewhere to start and then through a little bit of trial and error ...
... take a ride to see how it feels, too much weight and the bike will feel bad, too little and the trailer will want to sway all over the place.
... shuffle the load inside the trailer, you'll likely find a sweet spot.
... If your ball weight is too low, the trailer can develop a resonant sway, which will increase until you crash, which won't take long on a bike.
Yes, that could be a very, VERY bad outcome. However in my experience (with the CB1300 and Classic trailer), an alert rider will feel the trailer starting to sway way earlier than a driver towing a caravan. I put this down to the inherent instability of a two wheeled tow vehicle when compared to a four wheeled vehicle, and most riders being very much "in one" with their motorcycle and will pick up anything out of the ordinary very quickly.
Tyre pressures can also have an impact on sway. Before you start shuffling your load around inside the trailer to eliminate sway (if any), make sure you have these as per the manufacturer specs.
You also need to be aware that too much mass on the tow ball can induce a bending moment at the point where the vertical supports (on a Classic tow hitch) join the horizontal bars (I have some pics which I'll post, if I find 'em). On the underside of the horizontal bars there is a longitudinal plate with a number of holes to attach the vertical supports. When possible select the holes closest to the the tow ball, but this may also have an bearing on the tow ball height, so a bit more trial and error may be needed on this point.
