Hi, I don't like roundhouse kicking with the ball of my foot yet it is the technical way that it should be done in my TKD and that is what I have to do for gradings and stuff but for sparring or in karate - my preference is definitely the instep of the foot, i like to throw the kick from my rear leg and just once if hitting hard but making sure the kick is really powerful or a couple of times off my lead leg if trying to catch out/score points in competitions. hope this helped in anyway