I'd love the authenticity of him just being an American, and born/raised in Nebraska but with a british accent just doesn't make real sense. There's no logical reason Marvel's Stephen Strange shouldn't have an American accent, and I'm sure BC would pull it off as well as any British actor could.
But holy Christ does his voice sound awesome in his native accent. His dramatic scenes, in his own accent, actually make me question what the limits of acting are on people's emotions.
It would be more authentic for him to be American. It's definitely what he SHOULD be. Dr Strange is an American, born and raised.
But I think it may be better for the character and his impact on the audience for him to be British. BC's voice connects people to him and to the scene on a deeper level than most any actor I can think of.
I'll be perfectly happy if he does the movie in American accent, but can't help but feel like I'll be wondering the whole time how much better the dialogue would be were he to speak with a brit one.