If you have her fixed, her chances of ovarian cancer would be 0. Her ovaries are completely removed in a spay surgery (which is more accurately called an Ovariohysterectomy).

Actually, whether she has puppies or not, every time she goes into heat, her chances of mammary adenocarcinoma (breast cancer for dogs) goes up. If spayed when young, before their first heat cycle, the percentage that develop mammary cancer is right around 0%. It also prevents a multitude of other severe problems like pyometra (a uterus that fills with pus due to abnormal heat cycles common in older dogs).

I would have to say you should have her spayed. There will always be plenty of great puppies available whether you breed her or not. She will be MUCH healthier and happier if she doesn't have to go through the whole ordeal.