most vets spay around 6months. Everyone has a difference of opinion on first heat or before. If I am not breeding I would prefer to do it before a first heat. I believe that a female could have the potential for more health problem related to homones if not spayed when young. I even spay my breeding females around mid age to help prevent unwanted hormonal health problems. uterine infection is one of my biggest fears. Although brest cancer is another biggie.
I would consult with your vet on what he/she believes you should do.
For a larger dog, yes, I would agree because they mature at a lower rate than a small dog but the OP's dog is a Shih-tzu so 6 months is typical for a smaller dog (who would typically go in heat sooner than a larger dog).
In order to really enjoy a dog, one doesn't merely try to train him to be semi human. The point of it is to open oneself to the possibility of becoming partly a dog. ~Edward Hoagland
Barligirl I understand your feelings about never wanting to have it done again after your traumatic experience. REalize though that it was probably the veterinarians incompetence that lead to the death of your dog and not the procedure itself. I know that that doesn't make things any better for you, and I would probably feel the same way and be scared to death to have it done on another dog, but I would still always recommend the procedure being done.
While not all dogs get breast cancer and pyometra from being unspayed, the risks are much greater to have them spayed if they get pyometra and if the condition is not recognized soon enough, it is always fatal.