I can only speak from my personal experience, but I know I have erred on the side of caution when it comes to which content label I and my partners give our threads. Understanding that NC is a very sensitive area for some readers, I have tended to place any plot which involved even a hint or possibility of NC in that category. Only if I am sure a story will remain completely consensual throughout will I place it in the Light section with a good conscious. I know the moderators can easily move a thread if needed, but, again, I would rather overestimate a story's provocativeness than underestimate and possibly offend or trigger someone.
For example, two stories I have currently noted as NC involve a character that is willing but pretending to be asleep and a couple who are a little bit rough (love bites, light scratches, etc.) but otherwise loving.
I hope that addressed your question a bit.