In my opinion and experience there is no reason for those with trait in threat reduction from flank/stealth to stay in that room under most circumstances.
Maybe pop in to revive or help but flank not only reduces threat but also scores big hits. If rest of the group barricades themselves inside the room, the one with flank advantage and priceless tactical view should stay outside, does not draw much threat anyway, and either pick high priority targets before they reach the room or just mop up. Hunter, for example, depending on difficulty can eliminate most of threats incoming, and in this sense can replace even AoE masters like Elementalists. The big advantage here is that is not .. red dots -> Firestorm!!! .. its picking them carefully one by one by priority. Those jumping demons messing up DC kiting/tanking? They will not need to get there.
So no. There is no one true way to approach 5/5. There are some principles but the zones do offer choke points, vantage points, and cover elsewhere than only in the entry room.