Well, on my main PC I have 2 160GB HDs split into 3 partitions each, making a total of six partitions! That means I'd need C-H: and my CDRW is I:, DVDRW J: !!!! 8 letters in total, 6 are hard disk only.
I'm guessing your optical drive is Z: ?? Why else is it there?
On my retrobox I have 3 partitions plus the optical drive.
Taking in to account that very few people will have more partitions than me, perhaps H: can be the limit?
If it's too much trouble, then just stick to what you want. I can always add the drive letter(s) myself due to my odd setup.