I don't think head faults are necessarily the kiss of death for a breeding career, especially in a breed like Shih Tzu (which, like collies, has a very specific head type that is the ideal, with MANY factors that can all individually vary- it's not like a Cardi head where you check off the boxes- ears at proper set & large enough and not TOO pointy, eyes not too round or too big or too small, and general profile okay).
All that said - health testing is a must. At least eyes and patellas!