It may be the trendy thing to advertise right now, but no- not all web designers code to standards. That’s why designers started making the distinction in the first place. Yes, it is very ‘this car does indeed come with wheels’ to proclaim that your code is valid. However it’s just another way of communicating that the quality of your work is set at a certain level.

Every industry has a different levels of quality, and subsequently different ways of communicating that quality. Apple does it, most designer labels do it, hell even Moleskine loves to stroke its own ego; does anyone really care that their covers are oiled-whatever-cloth?

Just make sure you actually educate your clients about web standards, otherwise they’ll have no idea just how awesome your wheels are.