Do you mean a front push kick? It just depends on the distance. Very close range, whole foot. Longer range, ball of the foot, because you have a much farther reach. In the end, it doesn't matter that much because if you have enough thrust from your hips the push will be strong enough regardless.
The heel is only really used on side push/thrust kicks and spinning/non-spinning heel hooks and turn kicks. That's just how I've learned, so I'd be interested to see if anyone does it differently.