I'm on the both bandwagon. I am of the opinion that no matter how well the two know each other, how attentive the Dom is to the situation or the level of experience, no Dom/me is ever able to know exactly what is going on with a sub totally. I have seen situations in which a scene went from being perfectly controlled and sensually satisfying to a situation that required drastic action within just a few seconds. A bad muscle cramp can be a serious problem depending on the situation and is almost impossible to see or detect by another person as an example. I always try to remind everyone that the whole intent is for everyone to enjoy themselves, to have an erotic good time and to be healthy and in relatively the same condition at the end as at the beginning. I think the concept of "total control exchange" is a dangerous concept that can lead the in-experienced into some really serious misconceptions.