When landing a teep kick, should one apply pressure via the toes/palm of the foot or the heel?
Either is acceptable. You will find MT and kickboxing coaches preaching one or the other, and calling either one terrible technique. Such is martial arts.
Using the ball of your foot is easier to land, your hamstrings don't have to be as flexible, and the damage is decent.
Using your heel will cause more damage, as you don't have the flex of your ankle absorbing some of the impact.
It's not a big deal which you try to use, and unless you're an experienced teep kicker you probably won't have much of a choice of what part of your foot you're landing with. This is all assuming you're throwing a teep to the face, chest, or stomach. If you're throwing to the thigh or knee I always try to land with the heel. The legs are too solid to strike with the ball of your foot. Your heel is a dense weapon that can really dig in to a thigh or knee.
We employ both in my training, but for the reasons coinbird describes the convention is the ball of the foot. I want to elaborate on their answer:
On top of being easier to land with the ball of the foot, it's also more versatile in where it can strike.
The foot will form a 'spearhead' using the ball, while using the heel does not (feel free to prove me wrong with evidence!). This could allow one to bypass a gap in a guard or strike a smaller area.