It's an option, but I did not have to do that as I installed IE8 after Visual Studio had been installed. If anything, I would uninstall IE8 and then reinstall it.
As for IE7 compatibility, I was suggesting this as something to check to see if you were still having issues with IE8 overall. Note that the purpose for the IE7 compatibility is because there are many sites that have not been updated for IE8. This includes some ASP.NET sites since IE8 wasn't available when the latest version was released.