There are definitely arguments for and against doing year round, but I go with year round. In the colder months I go the full 45 days between doses, in the summer months, I go monthly.
American Heartworm Society Updates Heartworm Prevention and Treatment Guidelines in Dogs and Cats I am in NYS.
I use Interceptor on my dogs.
I don't know how up to date this is:
Dog Heartworm Preventive Products Comparison Chart