I prefer to spread power-hungry devices (hard drives) across multiple power lines when possible. It shouldn't matter, because PC PSU designers fully expect folks to string multiple drives together, but I do it anyway.

My rationale is that distance from the PSU capacitors is significant. So with two drives on one set of wires, one drive can generate brief spikes/dips in voltage that could affect the other drive, due to their long distance (over the wires) from the smoothing caps in the PSU.

Pure superstition of course. Really.
But I do it anyway.

-ml