Oh yeah, if there's heartworm at all where you are, preventative isn't optional. And I say that as someone who also limits chemical treatments where possible (eg my dogs are only de-wormed if there's evidence of a parasite load, rather than just doing it routinely). The vet-approved preventatives are very safe; the treatment for heartworm is much harsher in comparison. Even for dogs without MDR1 to contend with. These worms have literally taken up residence in the dog's heart at this point, and while many dogs survive treatment, it's really hard on them physically. Even the 'fast kill' method requires a month or more of extremely strict crate rest/limited activity because of the stress it puts on the circulatory system.
Additionally if you give heartworm preventative as prescribed and your dog still manages to get it (which is extremely rare, to my understanding), these companies will cover your dog's treatment costs.
I'd still get her tested for the MDR1 mutation, since it impacts quite a few things as Shell mentioned.