I'm no expert,but I have cooked with a few.
If you think about it,the meat to bone ratio might be much less than on large chunks of meat.
Thus,a light ,even sprinkling,on each side,usually works.
If you are using much sugar,that is an extra layer.
Most cooks put it on about an hr before they go on the pit.
Much earlier,may cause them to cure and give you a hammy taste.
This allows them to get wet and the rub to stick,well.
Be careful,if you are using injected slabs,how much salt you add in your rub.