Since a shih tzu rarely tops 7 kg (16 pounds), it is imperative that you do what you can to keep them from being injured by what would merely startle other dogs that are even a bit larger. This is not because they're cowardly, but simply because the action of a falling toddler or a wayward book could cause permenant, perhaps fatal damage to such a small dog.
For starters, they should never, ever be left alone in the company of children. Even young teens who are normally very careful of the dog can turn incomprehensibly stupid when in groups. Smaller children have a tendency to treat these cute, cuddly, little dogs as if they were toys, since they really do resemble living toys. Of course, a living, breathing dog can't (and won't) handle the same sort abuse that a fluff toy can.
Moreover, your shih tzu may very likely get underfoot. They love to keep an eye on the leader of their pack, though any human in their house is an object of great curiosity to a shih tzu. Never, carry something large enough to obstruct your vision (such as a moving box) without first checking to see where the dog is. One wrong step and you could be looking at thousands of dollars worth of reconstructive surgery. When moving, people often make it a point to temporarily lock their shih tzus up in a crate while there's a bunch of dangerous furniture moving going on.
Along those same lines, it is good to get everyone in the household into the habit of checking beneath themselves every time they sit down in a chair. Even if the dog is supposedly not allowed on the furniture. It's better to be safe than sorry, of course, and you never know when your dog will decide today is the day to rewrite the rules just a little bit.
Some owners go to the bother of keeping a bell on their dogs ( as is the case with cat owners who are taking a toll on local wildlife), if for no other reason that to have an idea where the dog is at any given time. Of course, shih tzus are not bird dogs, so they will be far less resentful of the bell than most felines are.
Aside from the ever-present crushing danger, shih tzus are also small enough to be carried off by large birds of prey. While this isn't a concern for most city dwellers, a falcon or osprey is just as likely to pick up a shih tzu running in a field as a rabbit. As such, owners should never, ever leave them running around in natural areas without supervision.