I guess I'm going to go against the grain here a little bit.
I do think you have to carefully weigh all your options and make the best decision for you. Here is an article that talks about a study done on Rottweilers showing greater longevity when they kept their ovaries for at least 6 years.
GPMCF: Healthier Respect for Ovaries
Having said that, it is more difficult for you as an owner to have an unspayed female. You'll have to stay away from other dogs when she's in heat. It's said that stray males will be attracted to your home although up here in NY, I don't personally see that. Have not seen a stray dog here in probably 10 years and even then it was a loose dog and not a stray but that does depend on where you live. Females getting ready to come into heat can tend to be moody just like us females and if you have other dogs, that can sometimes be an issue. And of course you have links that others have posted. I just would encourage you to be extra vigilant and very responsible if you decided to keep her intact.