Is it important to have a traditional understanding of Buddhism or is avoiding so called "heresies" enough to make sure you are not distorting, undermining etc., the dharma?
Take the idea of an afterlife.
Religious and spiritual traditions throughout history have explained that death is not the end of life but that some part of us, perhaps all of us, somehow carries on.
Buddhism is no exception to this. It is undeniable that the historical Buddha accepted the idea of rebirth. He spoke of rebirth and frequently described, sometimes in considerable detail, how actions committed in this life determine the form of existence in a future world. He also spoke of enlightenment in terms of how many times one must be reborn before one will be freed from the cycle of birth and death. Although there are instances in his discourses (the Kalamasutta, for example) where he says that the practice of dharma is meaningful, whether you believe in a hereafter or not, the overwhelming mass of evidence does not suggest that he held an agnostic position himself.
https://tricycle.org/magazine/rebirth-case-buddhist-agnosticism/
I personally believe that we do experience the karmic result of this life, and that the next life is impermanent, illusory, but for whatever reason (mostly that accounts of past lives don't convince me) I don't think anyone experiences full rebirth as a new life. This is not traditional Buddhism. But as far as I know it is still a form of Buddhism and not a heresy. This is not Ucchedavada or the Senika heresy, I think, despite seeming possibly like either.
To an extent, it doesn't matter at all if you're not part of a church or pushing your idiosyncratic beliefs down anyone's throat, but is it dangerous, for you or anyone else?