Tickling is actually why I think everyone needs a safe word.
Even my most vanilla (female anyway) friends have had a partner tickle them until it was hard to breath while shrieking "stop, stop! OMG I'll kill you. I'm gonna pee myself" and the like. Yet it is to be understood that it is ok, and they like it overall.
You don't need to be tied up an beaten or participating in "rape play" to need a way to tell your partner absolutely and immediately that you need the play to stop.
Hell, the only time I ever used my safeword was when Stephen and I were having perfectly "normal" sex, but I started having an asthma attack. Getting the safeword out got him off of me immediately so I could go get my inhaler, where as saying "stop" might have made him stop with the sexing to ask what was wrong, but not have cleared me a path to my drugs.