I think it depends on the scuff as well...
Hammer Edge, I would scuff then knock. I find Hammer Edges are stiffer and a touch less sticky. So they are less likely to conform to the undulations after knocking in and don't stick as well in those spots.
Thinner self heal scuffs, I would knock then scuff. They are much thinner and generally stick very well. So they will conform and adhere well to an uneven, knocked-in face. And because they tend to crease and creep as the willow deforms, you will get bubbles along the edges if you scuff then knock.