The number one rule is "protect yourself at all times". It is your job to know your limits and to express them. It is your partner's job to respect those limits. When you feel in danger, or helpless, or lost, it is the time to tap. Only that will prevent injury.
This. I'm ultimately responsible for my body, and I'm the one who'll have to live with an injury. If I'm having an off day or am sore for some reason and tap "early," that is my problem, not my partner's. If I'm worried about my partner's control and tap early, it's his job to become trustworthy, not to tell me I'm "wrong."
I generally encourage beginners to tap sooner rather than later. Their partner may or may not "really" have them, but their first job is to protect themselves.
If I were to catch anyone in one of my classes ignoring a tap, I'd kick them off the mat. If they did it twice, I'd raise the question of whether they should be asked to leave the dojo.