I definitely feel that females that are not being used for breeding should be spayed but not before they are a year old. The chances of mammary tumors increases very little by allowing the female to have one heat but if you spay before a year older you increase the risks of other problems. The biggest risk for unspayed females is pyometra. If you have a unspayed female then you have to know the signs of pyometra.
Here is another point of view from another vet.
Castration of Male Dogs
There are risks not matter what you decide to do. I just feel that early spay and neutering in not in the best interest of the dog.