In the Hapkido I learned the heel for a mid-side kick, heel hook kick, front face/chin kick, knee after contact with the ridge edge. Back kick also is a heel kick, as is a heel down kick when an opponent is on the ground.
Instep for a face kick and some others. That may or may not be a classic roundhouse, since I was taught to try to raise the leg straight, and turn and kick to the face with the instep. I don't do that well. It's beautiful when done correctly since it is difficult to impossible to defend. Insteps can be used with inner and outer face kicks, but usually, the inner will favor the heel.
Leg strikes may be toe to the inner thigh, heel to the standing knee after the incoming kick is deflected with the instep, low ankle strike with the heel using a spinning kick droping to the floor, toe-ridge-heel, part or all to the shin in a outside to inside kick.
In the TKD I learned, most kicks were the toe or heel. Of course I only got to green belt, and I might have learned more later. I remember Mr. Rhee using a heel hook kick once that took us all by surprise since we had never seen it before.
All the above so say I think whatever works best is what to do, unless a form requires a certain part of the anatomy. Of course again, in the Hapkido I learned, making something work best was always the goal, if the normal way it was taught just didn't work for an individual.
