I've personally had all of my dogs spayed/neutered as early as vet recommended. Then again, I don't agree with unnecessary breeding, so I may not be the person to give advice. However, here are the statistics on Breast Cancer in female dogs:
"The risk of malignant mammary tumors in dogs spayed prior to their first heat is 0.05%. It is 8% for dog spayed after one heat, and 26% in dogs spayed after their second heat"