Determining hardness using teeth
No, teeth are not used to determine hardness. That would be incredibly stupid. What teeth are used for is distinguishing silt from clay.

When dealing with very fine grained rocks you might be interested in the relative proportions of clay-sized particles and silt-sized particles in the rock. This is very hard to do, but surprisingly quite easy when you try to put some between your teeth. Clays will have a smooth, paste-like, feeling, whereas silt will feel like fine grained sand. With enough experience you will be able to give a rough estimate of the proportions of the minerals.

Also notice this related question:

When can taste be used to help identify rocks?