It's not ALWAYS a terrible thing to talk politics at work. But to do it, you need to be smart. And that starts with knowing your audience before you step up to the microphone.
If you hang out with the same small group of people at work all the time and you're friends as well as coworkers, then knock yourself out. Whether you're all in agreement politically or you disagree but you know each other well enough not to be offended, then there's nothing wrong with talking amongst yourselves. It's a good idea to make sure no one around you is going to notify human resources because of your conversation, but if you have like-minded co-workers or bosses who you share a certain comfort level with, you should be in the clear.