I'm with Lisais mine on this one - safewords are fine - but as the Dom, I need to pay attention and watch how she's responding - not just emotionally but also physically.
Case in point: We tried a new toy tonight - my girl enjoyed it immensely - but I stopped play despite how much she was enjoying it because it went from making her bottom a lovely red to deep bruising in one fairly gentle stroke. There seems to be one spot on the toy that is much harsher than the rest.
She would have kept on receiving strokes - and suffered some deep damage if I'd depended on her using her safe word. The way I see it - her safe word is for when I misjudge and take her further than she can go - but it's up to me to keep her safe and make sure she doesn't need to use it.
Based on my observation and her comments just now, my girl would be more likely to respond badly (use a safe word) at the start of a session. Once she's past the initial few strokes she is deeply into it and feels sensation, but not pain as such.
So to get back to the original question - yes - I'm very aware that sometimes a safeword should be used and isn't - with my girl - when she's in her space the last thing she'd ever think about is a safeword - so it is vital I know her enough to know when she needs me to stop without her necessarily being aware that she's reached that point.