i asked a question here that got a lot of answers for the whys and why nots of spaying and neutering
http://answers.yahoo.com/question/index;_ylt=AnxG8cmrg1UfgsVviffRc1fsy6IX;_ylv=3?qid=20071030082325AAbBCGu

My own personal story, I didn't neuter my first dog because we had no other dogs and he was not running off anywhere, thus never mating. But midway through his life he developed cancer because I didn't neuter him and by then it was a combination of age and being too sick to put him under anesthesia to remove the tumors.

Also you can never know if your shih tzu will run out the door one day, even if you are the best owner and he is the best trained dog, he can escape whether its because he has a desire to mate, or just chase a squirrel. 10-12 million animals are euthanized in shelters in the US each year because of overpopulation. It can never hurt to have a healthy dog fixed. Their personalities are better, so you're happier, and they don't feel that forceful instinct to mate, so they are happier.
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there are no cons to neutering your dog….unless you consider preventing several types of cancer, preventing unwanted litters of puppies and preventing marking on your furniture, cons.

i certainly don't
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I used to have a shihtzu, I desperately wanted to have him neutered. Animal services didn't want to do it because they worry about respiratory problems- but the private vet saw no problem with it. If you don't neuter him, be prepared for him to pee on everything, and I mean everything, don't let him outside alone, ours would dig holes under the fence to chase other dogs, they are also at a much higher risk of testicular cancer when left intact. Unfortunately, our temporary boy went back to his real owner (my mother in law) before we could get it done.
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Regardless of breed, I think it's fine to neuter/spay AFTER the dog is an adult. Many studies today are showing increased incidence of disease in dogs altered at an early age, such as hip displaysia and cancer, which I am battling in both of my dogs who were neutered at 5 months. Sex hormones play a critical role in controlling growth hormones at the appropriate time. I will always neuter, but never again until adulthood is reached.
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I hope you don't mind me answering this post. I don't have a Shih Tzu, nor do I have a male dog. I just wanted to point out that it is important to spay female dogs BEFORE their first heat, which is usually around 6-8 months. For females, each time they go into heat their chances of breast cancer get higher and higher. I work at an animal hospital and it is always encouraged to spay healthy female pups at 6 months of age. However, I have heard of several doctors waiting for male dogs to reach adulthood to neuter them, regardless of the breed.
There is always a risk involved with anesthesia, but in regards to spaying and neutering, there are very few cons. Speak with your vet if you have any concerns. Good luck!
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