I like this approach: http://morten.dk/blog/ie6-tax-now
Normally I would say that IE6 and IE7 is not supported by default, but the client can pay an extra price for supporting legacy browsers (the website will cost 30-50% more depending on the website requirements).
It might be also a good idea to get information from similar sites. One of our newest deployed site has 2-3% IE visitors (including all IE versions), and the IE6 and IE7 users are below 1%. So we decided not to support IE 6 and 7 at all (we don't test them), and we give full functional support for IE 8 (the site is usable, and looks OK, but it isn't as sexy as in a modern browser with a better CSS3 support, which means that there aren't any gradients or rounded elements). But this site is a little special case.
So I say that analyze what kind of people will visit your site, and make a decision based on that information.