I carry my CP card with me even though I have never, ever been asked for it. I figure I would much rather have it and never need it instead of being left in a tough spot if somebody suddenly demands to see it.

Also, I get a little worried that they might ask to see it at one of the international stations where there is contract servicing handling one flight a day that may not be familiar with stuff like the CP. Other countries seem to like to see stuff like that on paper (plastic) in my experience.