When you are a website developer, hardly ever do you get to mandate what browsers your users will use. At my last employment we built a lot of Intranet web applications for our users, who all used the same version of IE that was managed automatically on their work computers via Active Directory. That was nice.
At my current position the websites I create are used by a variety of school districts and the public. While the majority of users are browsing my sites with IE7 and IE8 there are a few Chrome, FireFox, Safari and, yes, IE6 users.
There is a great community of software applications and web services to help you test how your site will react with different browsers. Most of them are fairly static showing you how a page will render, but with highly dynamic sites you just need to see how it will work on the actual browser.
Fortunately Microsoft makes this really easy by releasing VHDs with various operating systems and browsers loaded, from WinXP and IE6 up to Vista and IE8. That’s not the whole Microsoft gamut but it is a good portion and easy to use. Just load up Microsoft Virtual PC and you are set to go.
Take a look at Microsoft’s offerings here.