We have a tibetan spaniel that fits into the groups of breeds which have an underbite...just like your shih tzu.
There's good information on this mouth/tooth type at www.dentalvet.com/patients/orthodontics/pet_orthodontics.htm
Scroll down to the section that starts 'An underbite...'
You will see that this article refers to shih tzus among the examples of breeds which have an underbite.