I would suggest that it depends on where the knee is. If the knee is moving up into the area of the body I believe one has every right to hit the little bastard as hard as one can. 'Get below waist height you insurbordinate little git' is what a fighter should be telling a rapidly rising knee. If that knee doesn't listen then it will be getting a bopping. Secretly every opponent secretly desires to set loose upon the knee. Well, that's my tuppence worth.