Nigel's right that W3C compliance by itself is of no positive help. What it will do is let you catch HTML errors that might reduce your rankings.
For the most part I agree with the above responses. Google does not check if your website is w3c compliant. By ensuring your website is w3c compliant, you may be able to catch errors that are preventing the Google spider from properly crawling and indexing your pages. If w3c only detects minor problems, fixing them won't have any effect on your rankings. One benefit of w3c compliance is that it ensures that everyone can access your website regardless of what browser or operating system they use.
its just a tool to check on any errors, however i see it as the best tool in its kind. Therefor its been very helpfull to me.