I have thought on this too, I think it is best to treat for a specific problem if it crops up, as if you over medicated or medicate when not needed ,could just make the illnesses stronger if it strikes as it can get immune, I think it is best to build your birds natrual immune system by having healthy practices and good feed. and treat as needed. now for breeders, I think they should be medicated for canker before breeding. JMO