It really depends. The best principle to approach the question with is to match weapon to target. In other words, does the choice of contact area give you a better advantage or outcome. Different targets are going to demand different impact areas. So it's hard to nail down an absolute answer. That said, there are a couple of generalities that I think are useful. First up, the round kick, I think the optimal impact area for this in MOST circumstances is the shin. It's nature's baseball bat and has not little bones that are prone to breaking. It's also an easier motion for most people vs. the ball of the foot. However, I'd not say the ball of the foot is without use. Front kicks kind of belong to the ball of the foot. The trajectory just doesn't lend itself to either of the other options. The side kick is the heel or blade of foot, with again me leaning to the heel since it's less prone to damage.