I keep mine next to my drivers license and its visible when I get carded, no one has ever said anything other than a co-worker who saw it when I was getting money out. He asked me a couple questions and said he wanted to get his when he turned 21. A lot of guys mention they like to keep their hidden, but honestly most people don't look at anything other than the date and are to oblivious to notice what it is that you have.
I'm a server and there have been several times when I have carded people and they have both their HCP and license visible, I've often wanted to strike up a conversation about it but I don't because some guys would be upset if I asked about it in front of who ever they are with. Some people with HCP's don't want anyone to know they carry so I don't ever ask.