John,
Between dives one day I just happened to look at my better half's mouth piece and I was shocked! It was only about 6 months old and was almost chewed in half! In contrast, mine is not over 2 years old and looks brand new.
Turned out that Barbara had had her mouthpiece kicked out by another diver once and she did not want that to happen again! So she clamped down! At any rate, what I think I have learned is that if you are confident that you can recover a kicked-out mouthpiece, you do not bite so hard. So, my feeling is that novice training should include many repetitions of recovering a "lost" mouthpiece.
Of course, the irony in all this is that if you bite so hard regularly and do not montitor your mouthpiece, you may lose it buy biting through it!
Joewr