Excellent question Headcase! As you, and probably many others know, I OC as many places as I can. But there are those times when I think CC is the better option.
I CC to church for regular services. I do OC in church for other stuff that is not a regular service.
I CC for doctor appointments. There are people there who are sick and hurting, and causing them stress is not worth the issue.
I will CC if I am going into a section of town known for its bad guys. I want to promote OC as the norm in normal society, but some parts of town are not normal, and I do not need to be a target for someone who has something to prove.
Other than that, it is out for everyone to see. I just OC'd a little while ago into a local bar/pizzeria to buy dinner for the family. I got a bulge eyed look from the bar maid, but no comments, and no one had to be shot.