Whenever people have the "when to spay/neuter" discussion, someone refers to the studies that have been that show that dogs who are spayed/neutered early are more prone to certain injuries, illnesses, cancers. I am not disputing the validity of these studies, but I would like to point out a few things.
The incidences of these conditions are very minor in the first place. They don't happen often in an intact population so when the studies say the "chances of X happening are greater in castrated populations", it is important to evaluate HOW much greater. For example, if the chances increase from 1% of the population exhibiting the condition to 2%, then technically the chances doubled--but realize that 2% is still low odds. Now, I'm not sure what the exact statistics are, but to make blanket statements without looking at the actual empirical data is irresponsible and only contributes to misconceptions about a medical procedure.
An additional point to consider is that most pet dogs do not engage in the work that their breeds were designed for. In most cases, people are not talking about their agility competitor, working herding dog, search & rescue dog or anything even close. The most important factor in preventing injury in dogs is not spaying or neutering later in life, but proper conditioning, exercise, and nutrition. A dog with perfect proportions and conformation but that is in poor condition is still at greater risk for injury than a well-conditioned dog with conformation abnormalities.
Another consideration is that there are literally thousands of mixed breed dogs out there competing in dog sports and other physically demanding work. These dogs are of mixed parentage and, as such, don't have perfect proportions or optimum relative bone length ratios. These dogs perform their jobs very well for their entire lives. It would be interesting to see what the incident of bone growth related injuries are in this population. I would bet that the numbers are very low--in fact I would be willing to bet that they are very close to the numbers for intact, perfectly proportioned working dogs. I am willing to bet this because, again, the major factors for reducing injury in dogs is proper conditioning, regular exercise, healthy weight, and nutrition.
The last point I will make is that age at spay or neuter isn't the only thing affecting the overall growth and development of a dog's bone. Genetics and environment play a big part as well. I've seen plenty of intact adult purebred dogs with less than perfect conformation. If we use the example of stifle angulation that another poster mentioned, it is equally possible that an early spay of a dog that had less than ideal conformation could change the angulation of that dog's stifle for the better.
I'm not going to tell anyone when they should spay or neuter their dog. It is up to each individual to do their own research and discuss the different options with a vet that they respect. For me, the benefits of spaying/neutering a dog far outweigh what I consider minimal risks. After my own research, I've come to the conclusion that for average pet pups already in a home, spaying/neutering between 6 and 9 months of age is reasonable and in their best interests (10-12months for giant breeds). For pups in a shelter or rescue whos futures are uncertain, pediatric spays are in their best interests. For dogs that will be performing work or serious competition, neutering later than 9 months based on the rate of bone growth for that particular breed is in the best interest of the dog.