with a male I agree that it can be better to simply wait until it is necessary if at all. With a female it is different. Each heat can raise the risk of mammary cancer and tumors. And there is the inconvenience of having to restrict what your female does at least twice a year for about a month each time. Still, if you can wait until a female is full grown that might be the best answer.
Fixing your bitch for convenience is a personal choice. Quinnie is currently in heat. If I let her in, she will have to wear a diaper or she will bleed all over my bed as she did last night. Various canine sports, training, etc., will need to be postponed for a couple of weeks. That is up to the owner and no one should feel negatively about their decision to spay or not to spay.
As for mammary cancer, well, we are taught to believe that it will prevent it. I don't know what the risks are, what the numbers are of mammary cancer. Have I heard of it, certainly, but from no one I know. I have never spayed a bitch by first, second, or third heat, and have never experienced mammary tumors in my bitches. Nor pyometra, which I have heard of first hand. I expect it is just a matter of time, IF what has been promoted was based on fact and not just to promote altering dogs.
I suppose that makes me a bit cynical. But I have a real skeptical feeling of increasing the risk of some pretty bad cancers by doing something totally un-natural, to prevent something that may or may not be a true health concern facing all bitches. My grandmother and my aunt had breast cancer, but I am not cutting off mine to prevent it, nor am I cutting out my womb and ovaries to prevent it. There was one rather small study in Rottweilers that suggests that the longer one maintains their pieces parts, the longer their natural longevity. And doctors do not alter women willy nilly either. They leave an ovary if at all possible, even if other stuff must be removed for a medical reason, because hormones are for more than reproduction.
So if you don't want to separate dog and bitch, or you don't want to deal with messy heat cycles, or don't want to put your bitch on the shelf for a few weeks -- don't have the luxury of switching your training to another critter during her heat cycle, than that's up to you. For health reasons, I think the jury is way out on that.