I'm not convinced that IETester changes the underlying APIs correctly. Because of bad experience with "Multiple IE" simulators in the past I don't test on anything but the real install.
IETester hooking is not perfect yet, but the engine is the correct versioned engine used on each tab. You can find more information here :
http://www.my-debugbar.com/wiki/IETester/HowDoesItWork
Regards.