We have safe words and strongly believe in their importance. I have yet to use a safe word during play. I think of it as more of an emergency word for when something goes really wrong. Thankfully that hasn't happened yet.
I do yell and cry out words like: No, stop it, that hurts, please no more!!!
What happens usually is that my Dom backs off because he knows my words and actions well enough now to know when he is pushing me to hard. Granted there are times when he continues and I take it. Once in a great while I have come close to safe wording, but like many submissives it's not a word I want to have to say.