For subs that are gagged, I tell them to make V-signs with their hands if they are in trouble. I would use the thing-to-drop if I were trying something very edgy where they might be too confused for finger signs.

I would always start a new relationship with safewords/signals because I don't know the sub and might not be able to read hir signs of distress. It's always a big step in the relationship when I tell a sub sie no longer has a safeword. But even with a well known sub, I would have a safeword/signal if I were doing something new or risky.