Low frequency sounds do indeed carry farther, but they're also a lot less directional. This means that, theoretically at least, rescuers might have a slightly more difficult time pinpointing your location. As a practical matter, I'm not sure this has any real relevance in the case of the Fox whistle, as even a 'low pitched' whistle is likely to be fairly high pitched. A small whistle simple wouldn't be capable of producing a very low-pitched sound. I wouldn't hesitate to carry it.