Check out http://slingfive.com/pages/code/browserCaps/ for information on how to modify ASP.NET to recognize the advanced capabilities of other browsers. By default, ASP.NET does not recognize that other browsers can do the same fun stuff that IE can do. I have not tried this, but it looks promising.
I use IE 6 as my main browser. However, when I design CSS, I usually design against Firefox and Opera because these browsers are more standards compliant.
Once I get it right in FF and Opera, then I can start tewaking for IE, but keep in mind that you should test every tweak in FF and Opera to assure that you didn't break anything.
You may want to add IE 7 to the inital design target along with FF and Opera because IE 7 has good CSS standard compliance. Personally, I haven't added IE 7 as a target browser simply because IE 7 and IE 6 cannot co-exist on my machine. (Can't afford another PC and don't have enough memory for any kind of VM solution.) :(