This is all based on the assumption you actually have full gear on RDM once you reach a chariot I think? Personally whether or not I would cast in skill gear vs MND gear would depend greatly on how much I had unlocked.
In my group we gear up the rdm after the 3 melees, this makes boss tanking easier (paralyze, slow). I'm usually close to fuly unlocked as well as the whm and the brd in the "main" party. I probably use like 3k MP on a boss fight.