I would have to agree on this with what damyanti, fetishdj, and Ragoczy. A safe word, from initial meeting to years down the line, is a necessary part. Especially like fetishdj said with roleplaying.

As for what you were saying about "I know that there are couples in M/s relationships that say that the slave has no right to use a safe word, since the slave has no rights and has surrender all control to the Master." That sounds a lot what I was reading about in a different forum where the slave is no more important than a nice whip, flogger, etc. I know there is a name for that type of lifestyle, but I think that is taken too far. A slave is a person and deserves respect, and you should not get angry or vindictive if they feel they need to use a safe word.