Years ago we used a Hartz topical on our dog and it caused seizures. I think they reformulated after that.
My holistic vet actually said there is some evidence that dogs on healthy diets may not develop a flea problem. But he also said heartworm prevention in our area is necessary but only every 2-3 months. We feed raw but choose to use a flea/tick prevention because you can't walk out of the house here without being bitten by something. Ticks seem to like us and our dog regardless of how healthy we are. A lot may depend on the local pest load.