We always recommend complete adherence to the W3C compliance standards, it's a central tenet of our business. So we notice the W3C logo on websites that also claim to be compliant.
This is a great idea. It shows that you are a competent website developer, that you understand the importance of industry standards and that you understand the improved results that following these standards can bring to your business. Unless, of course, your website is not W3C standards compliant.
If any visitors click on the W3C logo on your website, it shows them whether your website is or is not compliant. It doesn't show them whether your website was compliant several months ago when you tested it and decided to put the logo on your website, it shows whether you are compliant now.
If there have been several modifications since putting the logo on your website and it is no longer W3C compliant, your visitors will see this in big red letters. This is very embarassing. Nobody likes people who don't practise what they preach.
This is even more embarassing if, as we have seen many times, you are a web developer. Several of our major competitors are guilty of this highly embarassing practice.
Now don't get me wrong, there aren't that many people that will click on the logo. But those that, are unlikely to be impressed if your website is no longer compliant and you are still claiming that you are.
If you are going to claim on your website to be W3C complaint, make sure that it is a policy to always be compliant and to re-check after every change.
All OpenGlobal clients benefit from specialist content management software that ensure that all updates and changes are always W3C compliant. There is no risk that "hand made" changes can introduce mistakes that break compliance.
If you are in the process of choosing a web developer, choose one that not only preaches W3C compliance, but practises it as well.