I've had mine frozen ever since the first year we, in NC, could do it for free. I've unfrozen a few times, but sometimes it doesn't work. For instance, I never succeeded in getting signed-up with bankofinternet.com, and although I unfroze, it still seemed to snag their system.
But I wouldn't have it any other way. Being frozen is the way to go. As Clark Howard says, the credit agencies hate it because they no longer can sell your information. So, since I'm a skeptical guy, I'd say if the agencies just happen to cause a glitch or two for people who have frozen credit, they're probably not going to put a whole lot of focus on fixing those glitches.