I agree with you. I never ask for safewords and do trust the domme to, as you write, step no more than a half step over the line. Also, if I were to be having some problem and just said, in a normal, non-submissive voice, "I'm sorry, but something is going wrong, we need to stop," I think the type of dommes I play with would get it and stop. Having the pre-session "here is your safe word" can be reassuring and fine, but I'm fine to not have that step.