I would say support IE 7 for now. The reason is for what you said, I think the pros who would hire just would want to know that you can solve issues for IE 7. Although, I doubt anyone of them uses that as their primary browser. I bet most are on Chrome.
IE7 is a lot easier to support than IE6 so its not really much that work to cater for it so I would say yes support it but not pixel perfect. Indeed ie8 doesn’t have most of the new css3 features either and needs to be coded much the same as ie7 anyway (apart from display:table and a couiple of other things although Ie8 isn’t as buggy as ie7).
IE is a pain… designing on a Mac it is decidedly even more of a pain. Fortunately ( I owe this one to Paul)… I code for compliant browsers/ add progressive improvement for bleeding edge THEN fix for IE. I have integrated this into my pricing model.
As you said it will only account for a 3-5% gain in users but showing you can support IE7 and even 6 may give you an EDGE over other designers. I make it a “premium” option, after all it’s only fair since it is more work for you.