It depends on your lifestyle. If your dog stays at home and doesn't go anywhere interacting with other dogs, your risk is low. If your dog goes to boarding, dog parks, play care, events, pet food stores, etc., you have higher risk as it's contracted from other dogs through the air. Most boarding facilities require it, so you can't avoid it if you need to go out of town and leave your dogs there.
Canine flu is a serious -- but not usually deadly -- respiratory virus. It resembles kennel cough but lasts for about 3 weeks. Most dogs clear it on their own...HOWEVER a minority of them do get pneumonia from it. That's potentially deadly and likely requires ICU treatment in a clinic with an O2 crate to save the dog.
Make the decision based on your risk of exposure, and your dog's health history. Dogs with other underlying issues that make them weaker worry me me a lot more for things like this. There are two strains of canine flu in the US, and the current vaccine covers both.
FWIW, my dogs have to be vaccinated because of boarding, and we've not seen any adverse reactions (even in my elderly dog or my GI-compromised dog who is sensitive to the lots of stuff). YMMV.