recently I had discussed this with a holistic vet in my area and she said it is a personal decision if you want to use it or not. Some dogs can still get heartworm even if they are on the pills. And there are less invasive ways of treating it if your dog does get it.
Also, here in Minnesota we were told we only need it during warm months. Usually begin in spring and then keep them on it one month after the first frost.
http://www.dogs4dogs.com/blog/2009/0...cation-safety/