Yes; slimline drives generally only need 5V, but I ran 12V "just in case" -- It's the same reason I ran a 5V wire up to where my screen is, even though it only needs 12V -- It turns out I may be having to put a USB hub in the front of the car, so the extra 5V line turned out to be a good ideea. That being said, the two ground wires in a regular drive power cable are actually the same, so there's really no need to actually run 4 wires - you only really need 3. I still ran 4.
I don't like to pull cables more than I have to. I ran 3 extra RCA lines between the trunk and front console, an extra firewire cable, and an extra 1/8" stereo miniplug cable. They're all unused at this point, but when I want an extra pair of wires to do something on, they're already there. I'd suggest over-cabling to anyone. It's a real pain to have to re-do.