validation is a great tool to check your code for errors. It allows you to track down specific errors and correct them.
It is recommended that you use a strict doctype to reduce the amount of 'errors' or redundancy in your code but it is really depends on the situation.
More over, a site that validates 100% doesn't mean its coded properly. You can get a blank document to validate 100%.
PSD CSS XHTML posted a fantastic template for a gallery. Not only is it purely css driven, sematic and clean, but it also has a lot of SEO elements most templates wouldn't provide. If it doesn't validate, big deal, make the changes and get it to validate yourself. In the end, he has provided this FREE for people to use.
Validation is not the be all and end all of properly coded HTML/CSS
