Letting HDD's power down/park...

I've never done this, I've always allowed my HDD's to remain powered up constantly. However my Mac has 2 externals connected and it would be good to have these shut down after X amount of activity (which I currently have it set to do)

I was wondering, is there any theories supporting or advising against this?

Do HDD's that shutdown and reinitiate have a lesser lifespan of those that are constantly powered?

What are your thoughts?

There is a counter in the smart controller that can tell you how many power downs a drive has had. but as long as the drive head is parked properly by the drive there should be no problem.stoping and restarting the drives