Found this article, in which aniseed and other things were tested to see what effects they had on buns...didn't read it all the way through (I'm REALLY tired), but thought it might help.
As far as I know, anise is perfectly safe. It is related to fennel, parsley and carrots. If I remember correctly, anise is also in the original flavor of Oxbow Critical Care. A quick check of some of my rabbit resources didn't even mention it in the safe herbs sections...more research is in order to get proof.
Fennel/Anise is totally safe. We feed it all the time!
Wetry to give the kids 5 different veggies a day. So we switch between that and celery in their veggie bowls!