Not to mention that modern virtual machine applications allow you to boot real partitions as a virtual disk so you don't even have to reboot in order to access any OS you choose to install. These days it's never a bad idea to give yourself a few options.
That said, if we're talking about forking over cash for both a Windows 7 and Windows 8 license, then perhaps you should consider one before you buy. You might not use both that often.
