Don't have a rib piece "yet" but have freinds with em. Yeah, they hurt, but if you can't handle the pain to have something meaningful and lasting in your life, then you don't deserve it... Just my thoughts on it
Basically the ribs/sternum/armpits are gonna tickle to say the least. But I had my back done in two three hour sessions about 3 days apart from each other, no real healing process.... And man that second session was a biotch! The skin was already inflamed and puffy/sour, going over it again with solid black definatly sucked. But I had no choice since I was deploying the next day, so I had to get it done quickly.
Don't do what I did... seriously, on a bigger piece have a 1-2 week long wait period for healing in between your sessions. Unless you like scar tissue!
Good luck either way, oh and DONT smoke weed while your gettin tattoo'd, it intensifies the pain since it's all your brain can focus on.