It really isn't hidden, just not lettered. If you want to be able to use the repair options without needing to boot to the Install DVD, it is good to have it. It does, however take up a primary partition, if that might be a problem.
If you have a problem and Win 7 insists on installing the partition, you might be able the use the Shift+F10 and run Diskpart to format the drive first. This is normally not necessary since there are disk options available.
Please note, I have no experiece with RAID installs. But make sure you have the RAID drives available, just in case.