I believe that for SEO purposes, CSS and HTML validation are both important. But it's not just the validity that is important, it is the size of the code. You should always try your hardest to compress your code so that it's less for the visitors to load and a bit less load on the server you're hosted on. A few KB here and there doesn't sound like a lot, but after 10,000 visitors it can make a difference.

When it comes down to it, as long as there aren't dozens of errors the most important thing is functionality. If your site is 100% valid but is scattered because of "incorrect" coding, then what good is the validity?