As a Google customer, we have access to a very useful set of wizards, which help us develop and maintain a healthy relationship between our blogs and the Google search engine spiders. Google Webmaster Tools helps us develop a sitemap to initially make our blogs more visible to the Google search engines, it provides us with tools to measure our visibility, and it provides us with helpful hints on how to improve our visibility.
Similar to Google Webmaster Tools, Yahoo provides Yahoo Site Explorer, which helps us optimise our relationship with the Yahoo search engine.
You can use Google Webmaster Tools for any active URL that you administer. If your blog is published to BlogSpot, you use the "xxxxxxx.blogspot.com" URL; if the blog is published to a custom domain, you use the primary URL for the domain.
Using Google Webmaster Tools for any web site requires that you are the administrator of the web site. Google doesn't want anybody who's not the administrator, of any web site, accessing search engine diagnostics for that web site.
The meta tag provided in the verification process is a token, and it must match the URL that you are verifying. If you change the active URL for the blog (rename it within "blogspot.com", or publish it to a custom domain), you will need a new Google Webmaster Tools entry, with a new verification tag. It is as important a data element as the token that you get in your email (a clickable link) when you're offered membership in a blog, or when you verify an email address, for an account (you click on the link, which opens the token, and applies it to your account).
With a Blogger blog in Google Webmaster Tools, you install the token into the blog template, to verify ownership of the domain (blog). Only a blog administrator has the authority to update the template. If you don't install the token into the template, you can't prove that you own the blog, and you can't use Google Webmaster Tools for your blog.
Elsewhere in Google Webmaster Tools, you'll find the Web crawl logs. Some information in the web crawl logs can be very useful to you, while other is informational only. It's to your advantage to separate the two categories. The "Preferred domain" setting in "Site Configuration" is a very small setting, that can have a big impact. And most of us setup our blogs on GWT so we can add a sitemap to the blog.