Personally I think it is a matter of first, weighing up where you feel you stand on the health issue, then considering what you feel able to do on the management issue.
One the side of early spay: There is a 7 per cent greater risk of mammary cancers if a spay is done after the first heat; this is well backed up by research. On the other hand, there is evidence and research that shows some dogs grow about an inch taller after a neuter done before the dog is fully finished growing. Some feel it is better for a dog to reach adulthood before cutting off the hormone supply -- and this argument is stronger, for me, that the height issue in causing me to weigh up what I would do. Some feel it may affect bone density but as far as I know there's little evidence on this and millions of dogs that have been spayed/neutered in recent decades have not created a higher incidence of bone problems.
Setting those issues aside, then one has to consider what you feel able to manage, and the risks of not managing carefully enough.
Many people find waiting through a first heat difficult. Others do not. But it does require very careful and cautious management where your dog really should not be taken out for walks or ever turned loose for a run, during the heat period. Also you need to be vigilant with doors, and in the garden -- a male dog (including large breeds -- boys don't care that the girl is a fraction their size!) will potentially scale a 6-7 foot garden fence to reach a female in heat, which they can scent a mile or so away. Females in heat are a major cause for intact males to stray and be lost forever, as well. Whole gangs of males will go for a female in heat of any breed and the result is often, not pleasant. Yet a female in heat may try desperately to escape and get outside, driven by Mother Nature and the very strong call of her own hormones.
If vigilance fails and the dog becomes pregnant? Well, you probably won;t notice this til quite late in the pregnancy then you face the dilemma of whether to do a late spay. And puppies that are most likely going to be a mixed breed if the pregnancy is accidental... are not easy to home and add to the pet overpopulation problem. The world simply does not need more mixed breed or poorly bred puppies -- check out your local shelter or Petfinder and you will find millions of dogs and pups waiting for homes and at risk of pts.
That said, I think *many, many people* can manage a female perfectly well until she is a year if they wish to do so. There will be a slightly increased risk of cancer for that female once she passes through that first cycle. She will also be able to complete her growing phse completely. Others will find waiting a challenge -- and most pet owners will not notice or care about a small bit of added height. Many people will know what is best for them and their situations. Others who are uncertain need to weigh up the two sides and think about what they feel comfortable with from all aspects -- Bruce's point!
I would actually question the bone density theory on the basis that it seems to go against what horse breeders have been doing for centuries. Males are gelded *in order to make them stronger, better jumpers, with more solid bones and better musculature*, according to a long article I was reading recently. Surely the proof is in the pudding -- if steeplechase horse breeders for example know from centuries of breeding that they get a stronger, firmer boned and better muscled horse from gelding a male, I can't see how it would cause weak bones in dogs. Though maybe there's a difference in male and female neutering...
Personally: I would spay a female at 6 months, or perhaps wait til 12; and neuter a male at around 8 months on. Incidentally, Leo and jaspar were neutered at around 8 and 9 months -- one is leggy and was always leggy (Jaspar) and coincidentally -- excellent at agility and jumps; while Leo is just as 'cobby' and compact as he was when I got him. Lily was apparently neutered before a year old and is tiny and compact -- if she grew an extra inch then she needed it as she would be very squat if she didn't have that inch! :lol: I think a lot of this is more related to the individual dog and its growth patterns than spaying or neutering myself, but that is based on my own anecdotal evidence which is totally unscientific. :lol: