My personal opinion, after doing much research, is to wait until the dog is a little older grown to spay, for us..we waited til Gucci was around 2-2.5 years old. The hormones are essential to bone growth and organ development, and the data that I was running into, is that by waiting til' after she had a few heats, I would decrease her risk of various bone disorders or deformities, arthritis and a few other things. I will say that taking care of a dog in heat is no fun and a lot of work, you really have to watch them all the time to make sure there aren't any accidental pregnancies, but also, the data I was reading was saying the longer you wait…the chances of various cancers increases, especially reproductive cancers.
A downside, The spay was harder on her as far as recovery than it would've been had we done it when she was a puppy, she was in a lot of pain, but my girl does not do pain well, she's a bit of a drama queen with even gas pains.
I'm glad she is now spayed and we don't have to deal with the cycles, and I feel good that I waited until she was older and more developed. I sort of paralleled it to doing a hysterectomy on a 12 year old human and how that would effect her becoming a woman and the essential hormones that assist in your body development. There is also some statistics out there that suggest that early spay/neuter can increase the chance of allergies and autoimmune disorders. She doesn't have any food allergies or sensitivities, just sneezes quite a bit when the pollen is very high ..just like I do