If you are serious about your blog or website, then you have to have an XML sitemap in the root directory of your site.
If you are running a WordPress blog, that can be easier than it sounds thanks to a free plugin called XML Sitemap Builder WordPress plug-in. The XML sitemap building plugin automatically creates a Google format sitemap and then updates it every time you write a new post or create a new page.
Even so, it has an option to rebuild your sitemap manually should you ever change your blog configuration, or permalinks, or whatever else you might update on your WordPress site. Since it is as easy as clicking on a link, I recommend doing it regularly whether you think you need it or not. It’s a lot better than logging into Google Webmaster Tools and finding a bunch of your links are coming up 404 Not Found or Unreachable, or whatever.