I have heard of Doms who force a sub to use a safe word (once or twice) - just so that the sub learns it is ok to use the safe word and will use it if necessary. That being said, I am against repeatedly pushing limits so hard the sub must safeword. A Dom ought to learn to read his sub well enough to judge when a limit has been pushed far enough without a safeword being used; otherwise, the Dom loses the trust of the sub. A sub trusts her Dom to judge how hard to push, wihout the sub having to "tell" the Dom every time by safeword. As you say, a Dom should know a sub feels like a failure when she has to safeword, something to be avoided. Moreover, if hard face-slapping had already been established as a hard limit, your trust was violated.