Im not sure what the OP is trying to say. So the problem here is when a tank engages a boss? I have yet to see a WHM or SCH pre-cast medica II or succor or whatever ability they have.

If you are trying to inform healers, the healer forum would be a better spot. It seems more like a healer problem, not knowing that casting before the tank engages is bad news. It also amazes me if for example a WHM would have medica II up before the pull. Easily solvable: "hey whm, if we have medica II before the pull, you will get aggro".. Done deal.