When you're testing for your CFI and II, there's not really a good reason *not* to take the tests one more time for the Ground Instructor certs. They're not really needed, but are useful if the school/FBO you end up working at does a full-on ground school.
Looking over greenhorn CFI resumes I always wonder about the people who don't have the AGI (and IGI if they're an II), if only because it makes me question their knowledge and commitment if they either a) didn't know you just have to take the same test bank a second time for the cert. or; b) were too lazy to do so. It's a no-brainer resume padding excercise.
On the other hand, they're pretty useless prior to the CFI level *unless* you already have a job lined up that will pay you to teach exclusively ground--which would be great experience. Not too many of those jobs out there, however, so do your research first.