I do things like that to my guys uncut cock. It's lots of fun. The 2nd pic is my favorite thing to do with his foreskin. Especially after he's been fucking my pussy and I've cum a couple times and my juice gets under there, I get to tongue it out like I'm earning a treat for my hard work.
I'm not rough about it though. I pull and play with the skin, but I don't bite or pinch. I keep a firm grip with my lips, but never risk cutting his cock with my teeth.