Safeword for "pracical" purposes, e.g. the bondage is hurting is okay, allthough I do think that ticklers will imediately hear and see if something like that is happening. However if one or the other do not trust the tickler to notice this, a safeword should be used.
Safeword for "having had too much" is, to me, not a good idea: giving the lee a way to stop the torture, ruin both my feeling of control as well as the lees feeling of submission. Instead a time limit can be set up, so the lee knows that there will eventualy be an end - but untill then she is truly helpless.