I'm fairly sure IETester is not a 100% accurate representation. Although saying that, I have used it before and it certainly is more convenient. I suppose it depends on how important the compatibility of your site is on the older IE versions.

http://stackoverflow.com/questions/5612170/web-testing-for-i...

This tool is not a perfect replica of older browsers. I've enountered JS errors that aren't consistent with genuine IE versions.

It's fine for working out layout quirks, but if you're testing larger websites I wouldn't depend on it.

It works about as well as IE9's browser emulation. Which is not very well. It's not 100% true to native IE versions.

I've used this, and oddly enough, I've seen issues that only happen in IE7 on XP...IE7 on Vista/7 worked just fine. YMMV.