Sign-off's are not required for any instructor written exam.
According to the CATS website:
http://www.catstest.com/html/applica...nstructor.html
you don't need any signoff, although FAR 61 leads one to believe that one is needed for the FOI ... whatever. I'm not sure where the exemption is.
couldn't find anything that superceded that requirement for people going to take a CFI written.
As for the other ratings, look under eligibility requirements. It'll say whether you need a sign-off. Not required for ATP, for example.