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I think that meta tags are important for on-page optimization. For youtube videos and blog entries though, they might help a little with your site's PR. It wouldn't hurt to insert keywords in tags, as long as they're relevant and not spammy.
Tagging yur posts are not only good, simple SEO, but really help your readers find posts that are related to their interests. After all, that's your purpose - to help them find the information they want, right? Why wouldn't you tag your posts?