IE in the past has not followed the same box model that other browsers have. I don't have extensive experience with IE9 and how well it now behaves in relation to the non standard behaviour of the earlier versions. There is a substantial 'References' listing at the end of this IE9 Wiki article [en.wikipedia.org ]
I myself design first for Firefox 4/5. Then I check for wonkiness in IE6/7/8 and Safari. I make allowances in the initial design knowing that all browsers have some box, padding, margin and outline default differences. I'm not a pixel perfect insistant designer, esp when it comes to IE6 or those on 1024x768 still.