I have never played with safewords, only in some rare circumstances of extreme pushing or RACK.

For me, I enjoy being dominant and constant control of the scene. Part of what I consider my craft is to closely observe my sub and see how is body is physically reacting. I find it brings me closer to my sub. For the sub, I would hope that he trusts me and instead of thinking about when he should say "Yellow!" or "Red!" he can fall safely into subspace bliss.

A hallmark of a good scene is for me to find the edge and half-step over it and come back safely.

I know others feel differently about this. Other perspectives?