I will say this...
Even my super hardcore raw people that are totally against vaccinating and flea/tick meds still give heartworm meds. Do you remember Hershey always saying you can just go buy the active ingredient and give a small dollop, that's what they do. I don't want to risk it so I give the meds (heartgard from the vet), but we move a lot, vacation a lot, hike a lot, swim a lot etc. Choice is yours. I think, even if I lived where you do, I would still do it. But maybe every 6 weeks or something instead of every month.
I know it is hard. We are making these choices for our dogs and if they get cancer or sick or whatever, we take all the blame because we make the choices. But you have to decide what is best for you and the boys, what's your risk benefit?
Good luck making your decision, Nancy. I know it's not easy.