I'm a newbie, but I'd say the obvious answer is both. When gagged or unable to speak, however, it's all the dom/me's responsibility. A good dominating partner has a soft spot for his or her sub, and I believe should know when things are out of hand. On the other hand, I've never heard of a sub who died because he/she could have said "Stop!" but didn't. They'll ask for it to end before it gets to serious, and there should be a safeword.