There are two schools of thought in this.
What seems more popular is that females have a far greater chance of getting mammary tumors if left intact after the first heat. There is still a good benefit if the dog is spayed before the second heat. After that, it does not seem to make any difference. Females can get pyometra, where the womb fills with pus and will kill her. The chance of getting this increases with every heat cycle, perhaps because she ages, and perhaps because she is more vulnerable during estrus to infection. Anyway, it usually hits a month after she goes out of heat, and the optimum treatment is emergency spay. Any time you spay a bitch as an emergency, risks go way up. Performing an operation in the presence of an infection is risky.
The less popular school of thought is that the risk of osteosarcoma and hemangiosarcoma go up with early spay/neuter. Also, the hormones that are removed are there form more than just reproduction. And it compramises the health and immune system of the bitch to spay her. Also, the operation itself can be risky as there is a requirement for anesthetic. I personally will not spay unless there is a problem.
Other legitimate reasons to spay are not wanting to deal with seasons, and having to take breaks in training, etc, dealing with broodyness. It is funny. My training classes are frequented by a lot of people who are breeders, showing, etc, and whenever any of our girls are really acting up, we say "maybe she is coming into heat." And sometimes they are.
Were I to spay, I would wait for the bitch to reach her full potential -- 2 years, hips and elbows exrayed, etc.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.