I'll make a reference to this thread, [webmasterworld.com ], where GoogleGuy (Google's representative to WebmasterWorld) responded to this issue.
I don't think it's optional whether you validate a site or not. I do think it's optional on whether you take the advice of the validator for specific reasons.
In Google's case, making their home page validate would add approximately 100 to 200 bytes to their output. Multiply that by 100 million, and yes that does turn into a huge amound of bandwidth for no obvious advantage. The things on their home page where they didn't follow the rules were fairly minor--missing DTD, some unquoted attributes, a missing alt attribute, and some missing type attributes.
But realize that Google is in a unique position on the web. As the #1 search engine, they don't have to worry about getting their site indexed by search engines.