Patching and Plastering - Complete Plaster job

I have been teaching myself how to plaster but still have a couple kinks to work out. I have a couple rooms to plaster which have been newly gyprocked. The main question I have has to do with plastering the seems. I saterted out with a 6 inch troul working up to a 12 inch, sanding after each of the 3 coats. question: where the 2 sheets of gyproc join they are tapered into each other, is the idea to fill this cavity so it is flush with the rest of the gyproc, or should the plaster be feathered out beyond that point???? Seems to make sense to me to bring it flush, but i've been told by a friend that it should be tapered further. Any help would be greatly appreciated.