For large breed females, since there IS an association between earlier spay and a reduced incidence of mammary cancer, I recommend these owners wait until the dog has gone through one heat cycle and then spay them about 2 - 3 months later. (k9handler.com)
This includes the fact that spayed dogs have a lower incidence of mammary cancer and prostatic disease and eliminates the risk of a deadly infection of the uterus called pyometra. (cincinnatilabrescue.org)
Spayed females have a lower incidence of mammary cancer than intact females if spayed before the first heat. (rottsnpupsrescue.org)