If you are showing in an informal class where a polo is also acceptable, I think a show shirt with the collar unbuttoned is fine. If it's a big enough class that you are going to put a jacket on in the middle of summer, your collar should be buttoned. If it's not too hot to wear a jacket, a buttoned collar for ~2-3 minutes in the ring won't kill you!
I wouldn't call that a popped collar. That's just a show shirt with an unbuttoned collar. I don't like the look, although it bothers me less without a jacket. I (personally) think it looks sloppy with a coat.
This is a popped collar -- it's what you do to a polo shirt: