Here is the deal: you gotta test in IE. You don't have to use it, you don't have to like it, but you have to test in it. But which version do you test in? A normal windows configuration only allows a single installation of Internet Explorer. So if you are the good little downloader that Microsoft wants you to be, you are running IE 7. The problem is that IE 7 isn't nearly as awful as the previous versions, so it doesn't make an ideal candidate for a testing environment. The more bugs you find and fix, the better.
In my opinion, if you have to choose, do your testing in IE 6. IE 5.x is a dying fast and it shares many of it's oddities and bugs with IE 6.
But I'm on a Mac! Good for you, me too, but that doesn't skirt our responsibility to do serious IE testing. If you have an Intel Mac, you are in luck. I whole-heartedly suggest getting yourself a copy of Parallels. Parallels allows you to run Windows right from within OS X without rebooting. I have chosen Parallels as a sponsor for this blog for this very reason.
Here is the good news: you don't have to pick a version, you can test in all versions of IE! Check out the Multiple IE Installer. This software allows you to install all kinds of versions of Internet Explorer for testing.
Now that's Internet Explorer testing! They do have a warning up saying that some version of IE running under this software are subject to random crashing. Not great, but really a small price to pay for such a great tool.
Thanks to CSSVault for making me aware of this.