every dog is different and you will get tons of different answer's. Some yes, some say it doesn't matter. really up to you if you are going to be diligent and NEVER leave your dog alone with another dog, even if the bitch doesn't appear in heat doesn't mean she can not get pregnant..
I would fix her before 10 months, hopefully before her 1st heat. Although alot of dogs do come into the first heat around that time so if she goes into heat when you are set to get her fixed I would wait until her heat is over a couple weeks.
Genes have nothing to do with spay and neuter. I honestly don't fix my bitches until after their first or second heat. But my dogs stayed inside all the time and wasn't outside unless with me on a leash.