If your going to bust, then tell her to back away or just ask. "leaving it up" means your probably going to make her get pissed off and you may never get the same level of service again. Or she won't trust that you have any "control". I had a regular who clearly understood my level of control and what I was comfortable doing and that I would never do anything intentional to hurt her, so her service level was crazy. It was relatively safe but her BBBJ was bascially up until the last minute and she would simulate FS way too close with her thighs for a long time.
I highly recommend that you don't accidentally bust in anyone's mouth unless they are cool with it.