Priority isn't necessarily the issue IMO. A drive can never sleep if it's constantly being accessed. The concept of an idle computer seems lost these days. I get antiviral software, but I've actively tried to disable EVERYTHING on my computer that would access the HDD just to see if I could get it to idle, and I was never successful. And I mean going down to using SysInternals (now Microsoft) filemon to see exactly what file I/O is performed. At a certain point, it just came down to the OS "doing stuff" and I gave up.
On my laptop using Vista, constantly accessing the HDD like that can't help battery life at all -- low priority task or not.