Many questions devolve from this simple one, IS.
I'd suggest before enquiring further that you do a little searching of the site on the subject of which part of the foot should be used for striking.
For me, it all depended on circumstances and target as to whether I used the ball of the foot, the edge, the heel or the arch to strike with. It has to be said that I've never used a roundhouse in a 'real' fight.
The one kick I have used there is a downwards front/side-kick (not sure in retrospect which it was (I think front-kick) but I struck with the heel) to the knee - I was wearing 'Cuban' heeled cowboy boots at the time and was horrified by the damage I did.