Safewords are not used to stop a scene because it "tickles too much." Safewords are only issued to my partner as a way to stop a scene if she is feeling pain or an unusual miscomfort - a cramped muscle, cold extremities indicating lack of bloodflow, or some other type of pain or miscomfort that is a detriment to the scene.
If I am playing with someone I am unfamiliar with, I may grant them a safeword if it makes them feel more comfortable. If they use it because "it tickles too much," they get a break - and when we go back to it, the tickling is even worse. 😀