I usually go half and half. I do about 5-6oz of cocao nibs in the boil at 5 minutes, and then 5-6oz of chocolate power in the secondary. Either way works fine. You'll get a more noticable chocolate flavor if you use it in the fermenter. In my opinion, if you boil it, it adds a more "natural" chocolate flavor. Sort of like using a lot of chocolate malt.