I copyright the majority of the photos that I post online (although sometimes here on PT, I get a little lazy!). But yes, normally it's a good idea to place a copyright or watermark across your photos when you plan on sharing them online, like on forums or services like Flickr. There are a lot of people on the web who do not know all of the rules about using images, but usually having a visual copyright ON the image itself will deter most people.