There is really no simple answer to this question. My thoughts are that it should be based off of your desired demographic. If you are designing a site for people who are a little older and will be using older machines you should probably keep hacking and make the site work in IE6 and other older browsers. But if your site is more high-tech and your user base is predominantly on newer machines then you might not have to support it. Eventually this whole conversation will be mute because eventually computers with IE6 will hardly exist and the newer browsers are built much more to standards.