So far, I haven't had any issues with Windows 8 preventing me from installing Windows 7. I actually went back and forth for a little while when I was having trouble with a SSD. That said, I have no idea what to expect after Windows 8.1 drops.
If the Windows 8 install gives you trouble, just wipe that disk. You can do this from a rescue environment, such as Parted Magic (partedmagic.com/doku.php?id=start). You just boot to the rescue disk and can work from there. It has options to format, wipe, and zero write the disk, in addition to other things.
However, if you are planning on doing any upgrades to your storage configuration, now would be the time to do so.