I've been using W3C since I started playing the web design game last year. What I
failed to take advantage of, was the very valuable
Link Checker they have on their site.
It will help you determine if you have any duplicate, empty anchor texts (which Google DOES NOT appreciate), bad links, errors and the like.
It is an investment of time, but one that will reap dividends for your site.
If nothing else, you can pat yourself on the back for "passing" if no problems are found.
I suggest running EVERY single page you have. It can find things easily overlooked, especially when pages of been changed, or used to edit/cut/paste for new pages.