In order to achieve good tongue posture, you must have proper head posture. When I chin tuck I feel most of the force coming from the lower third and some from the mid tongue. The tip seems irrelevant because while the lower third subconsciously applies upward force with proper head posture, this doesn't seem to be the same with the tip or at least not to the same effect. I think a lot of you'll agree that we should be focusing on head posture and full body posture and I think that in the grand scheme of things the tip of the tongue is irrelevant because things naturally sort themselves out over time if you just posture yourself correctly.
Tip of the tongue on the 1st spot is important in the end. I notice when I have my tongue on the 1st, 2nd, and 3rd spots, that my tongue stretches and uses muscles--including neck muscles--in a different way than when it's not on the 1st spot.
If someone can't reach the 1st spot while also hitting the 2nd and 3rd, then I agree that the posterior third is more important than the tip. But in the end, the tongue plays a crucial role in neck strength, and then in turn with head posture.
Posterior third is most important since it helps to counterclockwise rotate the maxilla