The time out period of a HDD is controlled by it's controller, this will be unique per drive, and nothing to do with the WiiM.
It would potentially be bad for the data if the WiiM cut the power to a drive before it was ready to unpark it's heads, so I agree with WiiM's solution.
If you remember the days of "it's now safe to turn off your computer" this is because Windows would tell the drive to cease activity, so the user could turn off the system.