When theming a Drupal site, I find that there are no sweeter words to hear from the client than "IE6 support is not necessary". Unfortunately, this phrase is far from commonplace. Currently, IE6 has about a 17% market share, so while it is shrinking, it is probably not small enough yet to ignore (but what a glorious day that will be).
Many themers I know (myself included) like to work with the latest browsers and build themes using the latest internet standards. In my experience, the vast majority of themers use the latest release of Firefox along with the Firebug add-on. Unfortunately, this works swimmingly right up until the moment when the theme is tested on IE6, at which point a themer's day gets much worse.
What's the best approach for developing for IE6? In my opinion, there are two methods. The first involves testing on IE6 right from the very beginning of the theme development phase. As bits are added to the theme, they are immediately checked and if necessary, fixed using an IE6-only CSS file. This method often means a much longer theme development time (not to mention a cranky themer).
The second method which I've adopted on my last several projects is to ignore IE6 during the initial development of the theme. Once the theme is complete and tested on all other browsers, I test it in IE6 and fix or remove issues via an IE6-only CSS file. The key word here is "remove". Often, I can save my clients time and money by simplifying the design for IE6 by removing (usually by overwriting) problematic CSS. Since IE6's share is down to below 20% and falling, the clients are often more than willing to accept this compromise in order to save development time and money. Best of all, it keeps themers happy!