Well, it all depends upon who/what your target audience is.

If your audience comprises of web-geeks who're hellbent on finding faults on your website, I'd say certainly stick to the standards set out by the W3C.

However, if you're catering to a niche such as cooking - and your audience is housewives, all you need to do is make the website appealing aesthetically and thats it.

Validation doesn't play an important role here.