Drupal is a well-written piece of software that produces well-formed web sites. However, don't assume that it will still be that way when you're done with it. Not all of the modules, themes, or content on your site will pass muster. This is especially true if your site is open to users to create their own content.
You should run a comprehensive scan of the site to check for improperly formed code, broken links, and other oversights that could hinder your search engine positioning. Obviously, Google can't reject sites just because they have bad markup (most of the sites out there have at least one thing wrong with them). However, bad HTML can confuse the search engine spiders. They're not as forgiving as a modern browser is to technical issues. By eliminating any problem markup, you can remove this concern from your site.
There is a great, and free, tool that you can use to scan your site. It's called the W3C HTML Validator.
