I realize this is a personal preference thing... and others have the rights to their opinions & kinks... but I don't like safewords and don't use them.
Why? Because for me, the lee ought to just be able to tell me if there's a problem using her normal language. For me, if they're pretending to be a character, (i.e. a character that hates tickling and tells me to stop, but we both know I'm not supposed to stop), it makes the whole session feel inauthentic. I'd rather her just laugh, squirm and fight to get away, rather than her giving an acting performance via her words that I'm not supposed to believe until a certain special word.