I would spin them down if they're up doing nothing for hours.
But if they're accessed every now and then (e.g. the root filesystem writes every few minutes something into the log files), then I would keep them up and running.
Think as well that if you keep something up for a very long time, like at least three or four weeks in a row, its mechanical parts will "get fit" to a "running condition", making them not able to restart after the first time that they are shut down and cool down. Companies usually try not to shut down their mechanical devices not just because of availability but as well because of this problem.