Depends on a lot of factors, especially the type of scene and how well the participants know each other. An advantage of agreeing a safeword, that many people miss, is that it allows the 'victim' to protest and beg for mercy, to stop! please no more! and really get into the role (as said earlier). Not every one gets off on being silent, or counting strokes in the classical style. I like to play out a 'NC role' the quotes inticating it is of course negotiated. that would be impossible if you couldn't protest, and since a protest would be quite rightly ignored, I believe it is essential to have a safe word unless the participants are psychic.
For others however it is important not just to feel powerless, but to BE powerless and step where there is no backing out. The top really does have to know and read his victim well for this.
Safety in BDSM, much like in real life, is a matter of compromise. The need has to be balanced with the risk, and I think is different for every scene, activity and participant.