Your dog has a better chance of dying from disease related to not spaying her, than to die while in surgery. It is a very quick operation, maybe a half hour or so, and not very risky. I highly recommend that if you are worried about her, to make sure they do pre anesthetic bloodwork and have an IV Catheter put in. An IV Catheter is like a portal directly into her vein, so if anything goes wrong they can quickly administer the drugs needed. Out of probably about 150 spays and neuters I've seen, I have seen one die, and it was a cat with low blood pressure. I've never even seen any real problems out of all those either. The only problem I've ever seen was when an unspayed dog got pyometra, which is an infection in the uterus and the uterus filled with pus, and they had to do an emergency spay to save the dog.

Please spay your dog. I had to help put down a dog with mammary cancer the other day, which she probably wouldn't have gotten if the lady had spayed the dog. When asked if the owner had ever been talked to about spaying, she said yes, but I just never "got around to it".