I just use windows power management settings to control my HD idle down. If you just go to the control panel, power options, and advanced power options you can find that windows can control everything, power management wise. I have 2 RAID arrays, and I believe windows will only look for a single array that is idle and it will power that specific array down once the timer has been met. So even though my OS array is unlikely to be idle, my second array (Data) can power down if it's not being used. If you have a JBOD array then each disk can power down independently.
I guess it depends on what your array configuration is because if you run everything off a single array (including OS) then it's very unlikely to be idle for Xmin or so. I would recommend using HD power down settings if possible, not just because of the power requirements (HDs use the most in servers, next to CPUs b/c there are no video cards), but mostly because of the excessive heat and runtime that is created by never allow the platters to spin down.
Every HD manufacturer has an estimation for each model that represents it's expected lifetime before failure (in hours).