Though HSS gives you a clear idea of where you stand on their rank list through their letters, they are not that helpful. First off, they send their letters right before rank lists are due. This means that it is not possible to tell another program it is your "first choice" until you receive your HSS letter (if HSS, for example, is truly your real first choice). By the time you receive your letter from HSS saying that you will "not be ranked" or "do not have a good chance of being matched," most of the other residency programs in the country will have already submitted their rank lists. Since many residencies take expressions of sincere interest very seriously, your early lack of interest may hurt you if you wait for that letter from HSS.
Hope this makes sense. Just thought I'd comment.