For me, I feel like I've done a good job if my code validates. Seeing the green check box on the w3c pages just makes me slightly giddy. As for group b, They usually only care that it looks and works the same across browsers. They only place I've found that this is not true is the government sector. They require complete validation not only with the w3c but also passing ADA tests (basically how does it sound with a screen reader).
p.s. when I say government sector, I mean specifically the state of California and a few counties inside it. I have had no ther experience with other government groups besides them.
