Google video about categories and tags
Matt Cutts from Google made a little Youtube video in which he discusses adding tags and categories from a search engine perspective. He says that Google can tell what your post is about, so tags aren’t needed. However, some aggregators use categories to show up in different feeds, so they are quite useful.
We build websites on Wordpress, which means search engines can easily find out what the crux of your website is. I’d like to reiterate that any posts that you write, must be written for your website visitors (and not search engines). If your visitors find tags and categories handy, say for navigating or searching for related content, then add them in a thoughtful way. Otherwise, you don’t need to ;-) Check out the video below.