I've been using straight ivermectin for years, every other month rather than monthly. A cardiologist i worked with many years ago told me vets recommend treatment monthly because people become forgetful on an every other month schedule, putting their dogs at risk by forgetting long enough for the difilara to mature, they mature in 8 weeks. So as long as the treatment is religiously given every other month it is effective - but you can't miss even one dose in this schedule because the difilaria will mature. If that happens and the dog is put on a MONTHLY preventative, the mature worms die out after 5 years, and new infection is avoided by the ivermectin.
The monthly schedule is easer for one or two dogs and if one dose is missed or given late, it's not a big deal just resume it.
HOW you give it should be based on a difil test, especially if the animals history is unknown. Infected - give monthly. Clear, either every month OR every other month absolutely religiously. An annual test is also a good idea just to be sure your protocol is working. Also remember H/W preventative also has strongid in it to kill other types of worms so you will have to step up that worming with something else.
We reposessed a pup once who was badly infected - had her treated to the tune of 1K and she ended up passing much younger than her littermates and never did really regain her "glowing" health. It was horribly painful getting insecticide injected straight into her flank muscles over the course of several days. I would never-EVER do that again to any dog, no matter what.