If the site's html and css are valid do i really need to set up links/buttons to W3C Markup and CSS validation services? Will they do any good things from Googlebot's (or others) point of view or only some harm because of pagerank flowing away? Thanks.
I wouldn't think it would have any effect on your Google ranking. Having valid html may well help because then Googlebot can better understand your page, but you don't need to link to W3C for Googlebot to know that it's valid.
When you say "pagerank flowing away", what you mean is "pagerank points that I could be recycling by sending to other pages of my site, I'm actually giving away to other sites". Linking out doesn't affect the pagerank of the page the links are on.
HTML validation helps in that invalid markup can sometimes prevent the search engine bots from properly parsing the page.
To give a simple example, a missing
</head> tag and the page would probably display just fine in most browsers - but the bot won't get the fact that the body of the page has begun, and all your precious keywords, <h1> and all the rest would be ignored. Validation ensures that your markup is not going to cause problems, and you can then concentrate on the other aspects of SEO.
CSS files are rarely parsed by the bots, so there is less of a problem there (although it is still better to validate - think of your users as well!).
However, having said that, there is no need at all to link to the validation services.