Validation makes me angry, like I don't get why <br> has to be <br /> or <img ...> has to be <img .../>
I don't spend the time validating. If I have 3 errors but the site looks fine in all browsers I can possible try, then I'm happy with that. I don't see how being 100% valid is going to make all my visitors super happy.
Its not, its for bragging rights imo.
It does help to fix some errors, not to mention you look like a doop to your customers if you can't even use proper HTML standards (<br />, <img /> etc.).
I do:
http://validator.w3.org/check?uri=http%3...tgo.net%2F
The only errors that website has are in the ad code, which I have no control over. I make sure everything is valid because it's the right thing to do. Forums are a bit trickier, though, because of MyCode - and because the default theme is rather out of date.