I can't find the article now, but I remember reading that the IE7 mode in IE8 had a ton of little gotchas. They were pretty much all the type of things that weren't a big deal on their own, but cumulatively it mad testing for IE7 on IE8 totally worthless. I think the tests would be unreliable, plus the amount of minutia you would have to mentally track about what is emulated properly and what is not was huge. My feeling after reading this article was that the simplest route, unfortunately is to have virtual Machines with the different browsers on them.