There's a lot of controversy on the subject of early sapying and neutering. Personally I think the only reason to do it really early is in the case of a puppy being adopted out from somewhere. It's really hard to check up on every single pup a spca or rescue has placed and the only way to be sure that it was done is for it to be done before the pup leaves the shelter.
Many reports say that the young pups bounce back from anesthetic much faster than older pups.

I tend to go with the school of thought that dogs need their hormones to properly develop. So this would mean letting a female dog go through her first heat, and letting a male dog fully mature. (I would say around 9 months depending on the breed).
While it may be true that early nuetering leads to slower closing of the growth plates thus resulting in a "taller" dog, I question how healthy that is. I wouldn't let that be the deciding factor of whether to alter early or not.

Oh one other thing. A reason many vets suggest spaying before the first heat is that many pet owners don't realize how carefully a female in heat has to be watched over. They don't think their perfectly obedient dog will stray, but the desire to reproduce at that time is so strong that many dogs will go over under or through a fence to get out. So that would mean the dog would have to be kept on leash every single time it went out for a month. Many owners don't want to go to that trouble so they alter before the first heat.

In the end it comes down to what you and your vet are most comfortable with.