Tone in any wind instrument is generated by a resonant column of air. The most important element of good tone in harmonica playing is breathing from your diaphragm and allowing the column of air - all the way from your gut and lungs through your throat, mouth and even nasal cavity - to vibrate and resonate. If that column is pinched your tone will suffer. But the physics of good tone don't change one iota if your tongue is touching the harp or not.
Peace and music, Dave