You can probably tell from my thanks where I fall on this question. I agree with the hard limits/soft limits distinction, if you're sure which is which, but I do feel that the Dom/me has responsibility for the sub's well-being - not in a global sense, but in the local sense that s/he chose to give up physical and psychological control of the situation and I think both aspects need to be protected. Otherwise, what's the point of the power exchange?

If someone is clear that there are activities that s/he is truly not interested in, I think even asking when s/he is that vulnerable is unfair.