I personally will NOT use straight ivermectin. Yes, it will prevent heartworms and many intestinal parasite. No, it will not do a thing for fleas. Ivermectin is what is in Heartgard. BUT, the dose you give with feed store ivermectin is much, MUCH larger than the dose in Heartgard. I have a collie I have not yet tested fro MDR1. It is safe to give him Heartgard because the dose is so small. So that's what I do. Based on his reaction ot anestheisa, he is very likely not even a carrier of the mutation, but I'm not going to put my dog at risk without knowing.
Even dogs that are not collies can be affected by ivermection. Mixed breeds with an unknown family history, other breeds known to have the MDR1 mutation can also be affected by ivermectin. We had a husky mix in a couple weeks ago, and based on HIS reaction to anesthesia he very likely is affected by the MDR1 mutation or at least is a carrier. You just can't predict it in mixes. He may not have had collie in him for many generations, but if one MDR1 gene continued to be passed along - there it is.
So, I guess if you really want to do this, and you have a breed ever affected by MDR1 or a mix breed, I would highly, highly recommend testing for MDR1 first.