All I can tell you is you should always design using Firefox as it is the most standards compliant.
Then anything that does not look right in IE 6 you can fix by overriding the css in the IESpecific.css file in your skin folder and you can correct IE 7 using the IE7Specific.css. So maybe you need to make column widths a little smaller in IE 6 to keep them from wrapping.
Generally there are less issues with IE 8 but in the next release it will be possible to include an IE8Specific.css file if needed.
Hope it helps,