I would absolutely keep my dog on heartworm medication in Northern California, all year round. We are in Sacramento and my vet, who is a solo practitioner, diagnosed 17 cases last year. Another member of this forum, that I believe lives in Auburn adopted a dog that later tested positive.
Even if, and that's a big if, there's no HW mosquitoes in your immediate area, they can travel inside a car or with a shipment of goods, etc. easily enough to infect your dog.