I know this is an old thread, but I once dated a woman who had inverted nipples. Hers would be best described at grade two because they would frequently get hard and stay inverted, but could also be coaxed into coming out with a little effort. They were surprisingly big when they were out. Her nipples were VERY sensitive. She would cum whenever I would repeatedly suck them out and push them back in with my tongue. Rough nipple play was a bit too much for her, but as long as I didn't get too rough she got intensely turned on by nipple stimulation.