Validation is a merely a tool to assist the developer in ensuring that his code meets the W3C Guideline no more no less.
When developing with word press (or any open-source CMS) build your basic theme and Validate without plug-ins deployed and minus content.
Then during the development process validate after each and every change to the system.
This will ensure that the basic system meets the guide lines.
Rogue Plug-ins
Should you require a plug-in that throws errors and the client doesn’t have the budget to rebuild, check the site with Lynx (text-mode web browser) with the plug-in deployed, if the content renders OK the plug-in will not effect the bots ability to spider your content.
Also check that the rogue plug-in doesn’t slow down your page rendering (as this can affect your page ranking).
Definitely No Holy Grail
Excerpt from Google Webmaster Tools
Clean, valid HTML is a good insurance policy, and using CSS separates the presentation from content, and can help pages render and load faster. Validation tools, such as the free online HTML and CSS validators provided by the W3 Consortium, are useful for checking your site, and tools such as HTML Tidy can help you quickly and easily clean up your code. (Although we do recommend using valid HTML, it’s not likely to be a factor in how Google crawls and indexes your site.)
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