I'd have one canonical one used everywhere, and 2n pure functions to convert at API boundaries. By making them pure functions you guarantee (or at least suggest

I'd have one canonical one used everywhere, and 2n pure functions to convert at API boundaries. By making them pure functions you guarantee (or at least suggest by convention; depending on your language's type system) that those conversions will have no side-effects. As methods, you can never be sure as the caller whether there will be ramifications to calling them in new places.