Any toy breed can have problems with retained teeth.
I googled "shih tzu retained baby teeth'" and this was the first result:
Shih Tzu Dog Breed Information, Pictures, Characteristics & Facts
Under the health section, it states:
Retained baby teeth and tooth and gum problems are not unusual because the Shih Tzu's baby teeth may remain intact when the permanent teeth emerge. Sometimes it is necessary for the veterinarian to extract the baby teeth. Because of the Shih Tzu's undershot jaw, he also can have missing or misaligned teeth. It's important to brush puppy teeth regularly and report dental problems, such as bad breath and loose teeth, to your veterinarian.
Here's another thing stated on that page that I've never heard, probably because I've never had a breed that is brachiocephalic:
Snuffles may plague the Shih Tzu because teething tends to be difficult. At about four months, the gums swell; since the gums are directly under those pushed-in noses, there isn't a lot of room. Puppies may snort, snuffle, snore loudly, or wheeze during this time, and may even have a clear nasal drainage.
It doesn't say how long the gums stay swollen, but if this is the case I sure wouldn't want to put a shorter-snout dog under anesthesia when they are not breathing perfectly. I may be way off base on this one, but why take the chance?