There is an incredible arrogance associated with this problem. I have contacted many site owners (some big name ones) in the past and told them they have serious browser issues and none of them have been interested. I have just dropped an affiliate product because his sales page locks up all versions of IE for about a minute. I contacted him and have had no reply in nearly a week, that's via his helpdesk so you can imagine what product support is like.
There are no excuses. Webmasters love to talk about how bad IE is and microsoft should sort their own problems out. This is not the way to do business. If your webmaster tells you your site doesn't load into IE6 because of the IE6 bug - sack them! It's a poor excuse for laziness. It doesn't matter who is right and wrong about browser compatibility, all pages can be made to work in all of the popular browsers.
Also, don't assume that because it loads fine on your own IE, it will do so on others. A page can load perfectly on a fresh installation of Windows but lock up the browser on a well used PC - the BBC website used to be a perfect example with this problem.
So don't forget to check your site not only for layout problems but also page load times. The affiliate product I have dropped is possibly losing 50% of his sales through browser problems while at the same time he is split testing landing pages to get his conversions up by a few percent. It's utter madness.
This site is useful for a quick cross-check, but don't rely on it as you sole test.
Check Browser Compatibility, Cross Platform Browser Test - Browsershots