If they're non-billable hours, you're missing a 'Full IE6 support is a chargeable extra, not core functionality' bit to your agreements ;-)
Edit: I'm surprised to be down-voted. Many clients that have standardized on IE6 still exists. 3 years ago there were even more clients that used IE6, and probably forbid other browsers, including IE7/IE8.
I've personally fought for making my web apps work on every reasonable browser, and gotten the arguments that the client will not pay for anything but excellent IE6 support.