It doesnt matter whether your website is W3C validated or not, but "Error Free HTML" could help you avoid pitfalls especially if you have several hundred pages on your website.
Take for example, if your developer forgets to close <h2> tag on several pages, then it could be mistaken as spam by search engines.
Therefore having a W3C validation is good, since your website displays evenly across all web browsers. Moreover, it means that your website HTML is error free.
It is upto you to decide whether you would prefer to have a W3C validated website or not. Personally, I would recommend to have W3C Validated website if you have big big website. Atleast it can help you make your website error free
