I'm in Florida, i've tried the 6 month heartworm shot before as well as the monthly doses of heartguard. I read an article some time ago, I can't recall its name, that questioned the necessity of heartworm meds. I hadn't given my dog any since and that was over a year ago. My boy has recently been tested for heartworm and was negative, the vet of course offered the shot again, to which I declined.
I'm not suggesting that others try this method, but I haven't had any problems. I'm still debating if I want to start the meds back or not. I pick up my puppy in less than 2 days, and that is one of the 2 remaining big issues that i'm wrestling with. I've already decided not to go with the neuter.