I've been sessioning almost as long as you have and that's been exactly my experience. Rarely given a safeword, I don't remember ever using it. I too have cut a couple (out of hundreds) of sessions short but that was because we just weren't a good fit, and had nothing to do with activities that might warrant a safeword.

If a sub is so nonresponsive that the Domme can't get any reaction to read, that's a different story. But those are subs few Dommes want to play with, safeword of no safeword.