Sheba was six and a half months when she was neutered. She needed some teeth removed but that wasn't what I was thinking about. I just did what I'd done with our other female dogs, who all lived long, healthy lives. I know now that there are good reasons to wait longer... until the growth plates have closed, I think? Someone will be able to give you a good explanation of that.
To be honest, I am grateful that we have it over with! I think I would be worrying all the time that she would go into heat without my knowing it. I definitely wouldn't be able to handle male dogs waiting around for her, or a litter of puppies. I'm just about keeping up with Sheba!
That is a VERY good reason for neutering early... And the one that vets worry about most. Unplanned and especially early pregnancies are a MUCH bigger health risk than early spay/neuter. So if you aren't POSITIVE you can handle a heat cycle without keeping your girl out of trouble, it is better to spay them early. You also have to weigh the risks of an extra anesthesia (for the teeth) against the possible risks of an early spay. While I have no fear that I could handle a heat cycle, I'd have to think hard about whether I wanted to risk a second anesthesia to do the two procedures separately. Fortunately, I have a great holistically minded vet who I know would help me weigh the risks and benefits of each option and then let me make the decision.
For most small breed pet-only dogs, it probably doesn't make a huge difference. If you will be putting stress on a dog's joints for sports, it is better to wait at least until they are fully mature... At least a year, some people say two. If I had a large breed puppy, between the joint protection of putting off spay/neuter AND the fact that those who are spayed/neutered later have less risk of some very bad cancers, I would CERTAINLY hold off. Fortunately, our breed has low risk of these cancers, but there are a number of dogs on the forum who have had CCL tears, which have been clearly linked to early spay/neuter.
The "best" answer and the acceptable risks are going to differ for every dog in every family. Read as much as you can before making your decision, but don't feel bad, whatever you choose, as long as it's an informed decision.