This is just wrong. If you install Windows via BootCamp, you do NOT need to re-install for Parallels. Parallels looks to the Boot Camp partition for use a the VM bottle. It's still a single instance of Windows.
Haven't used Parallels, but with VMware Fusion you do have to do the phone trick to get another activation key even though it's the same install. I think it has something to do with the fact that the motherboard is virtualised, but don't hold me to that.