Hi bearbrick, I think if you had asked this same question in the WordPress Forum you would have hard about various ways to control what gets into your sitemaps. The /tags/ directory is a virtual directory that does not really exist for example, it just gives people a different way to find your pages or posts listed by the tags you added. If /tags/ and /categories/ are in your sitemaps, Google will index those pages and view them as duplicate content.
Depending on your skills there are a number of ways such as the X-Robots tag that can send a message with particular URLs that the URL is not to be indexed. Many WP users resort to a plugin that lets them control what URLs get indexed. Overall, you want to have your Posts and Pages in your sitemap and not list your content any other way just because it is the same content, shown a different way. Some of these plugins also help with canonical links for the duplicate pages so even if they are found via links, it will not be seen as duplicate. I use Yoast's SEO plugin for that but it is not for everyone. You need to read how to set it up properly before using it.