Heartworm meds are TOXIC to your dogs. You can't know where a stray dog has been to get sick or what it has had to eat. I have rescued dogs and had to bring them back to as good a state as possible, due to the previous state they came from. It's heart wrenching to have to let one of them go, but they are better off if they are at that point. When you get your on dogs, you can control where they go, what they do and what they eat to keep them as well as possible.
The Whole Dog - sums up the logic regarding the needs of your protected pets and dog in the wild so you can make up your own mind how to treat your dogs.