Dual booting windows 7/8 with a view to upgrade 7 later
Solution 1:
Dual booting will work, but, it will upgrade the boot loader and make other system wide changes.
Quite frankly, if you have no intention of using this installation of Windows 8 long term, I highly recommend you instead look to virtualise Windows 8 for evaluation and test it in a virtualised environment as you should be able to do what you need without making any system changes.
Other than high end graphics, there is very little difference to using it virtually.
Just remember to go full screen otherwise you may have a hard time getting used to the gestures.
If you already have the ISO (from MSDN or Technet) then great. If not, you can get a preview here - http://windows.microsoft.com/en-GB/windows-8/iso After you have this, if you don't currently have any virtual machine software, I recommend you download VMWare Player
Solution 2:
I'd go with a VHD boot in this situation (which means you don't need to repartition) or go with this alternate install process , both of which keep the windows 7 bootloader, just to make the process of removing the 'old' windows 8 install cleaner. You'd just dump the partition/VHD, edit the bootloader and you'd be back to a 'pure' windows 7 install you could upgrade. Both options as I see it are cleanly reversible, and the only issue with a windows 8 VHD install seems to be the lack of fastboot support.
In most cases though, if you're doing an install windows would just note the presence of the old install and add it to the bootloader.