Where do you live? Most vets want you to continue these year round. I always stopped during Nov after first frost and started again in May. Some heart worm meds are also for fleas. Read the insert that comes with the meds.
I live in California and my vet recommends year round heartworm preventative. Most of them also Deworm against the common parasites which is nice and like mentioned above fleas. I don't see the harm in continuing all year and you know they'll be protected. It only takes 1 time to have your dog become infected and the cost to treat is much worse than the yearly cost to prevent.