When you install WP it comes with an autogenerated robots.txt file which is rather incomplete.
From experience I find this is the most appropriate robots.txt file (at a minimum), for a WP site installed right in the root folder:
If the WP installation is in a subfolder (example folder /blog/ ) then the robots.txt for the entire site has to have WP specific directives in addition to those for the rest of the site.
Under the general user agent (User-agent: *) line you will add these similar directives to the others that apply to the rest of the site:
In addition tag, category, archive and author pages require a robots "noindex" meta tag, because all they are at best is lists of links to posts, and at worst duplicate content when all all part of each post is listed as well.
This is easily achieved by using the All in One SEO Pack pulgin (or similar) and configuring it to add those robots "noindex" meta tag to those types of urls.
Configure the sitemap generator to not include those types of urls as well.
Note: In rare cases WP site webmasters are actually providing some good stand-alone content for each category page - In that case don't add a robots noindex meta tag to category pages.
Other things to watch out for when using Wordpress: