I don't know about Tansy, but here in the UK ragwort is considered a noxious weed. It is poisonous to livestock and spreads like mad if allowed to seed. Regardless of it's companion properties, or lack of, I wouldn't consider letting it get generally established anywhere.
Even if you don't have livestock now you may want some in the future, and you don't want your
land to be a source of airbourne seeds for others.
Mike