Consider this,
I have a 0+1 RAID setup with the 4 D740X harddisks stacked ontop of each other, maybe max 5mm between each disk. Without the cover on the case the disks get warm, but never hot! Okay, I put 2 fans behind this array to draw air over them and keep them cool when the case is closed and they're running 24/7 just fine thankyou.
Over the otherside of the office is a crappy Dell GX150 Desktop. Inside there is an IBM Deskstar running at 7200rpm that gets so hot I could fry an egg on it! Well, that computer has been running just fine 10 hours a day for 6 months or so now.
So everytime I buy a new 7,200rpm hard disk from the local computer store and the spotty git behind the desk tells me I need a cooler with that, I tell him no thanks.
In any event, those dumb fans which screw on the bottom of the hd are pretty much worthless. If you really think you need HD cooling then put it in a proper aluminium cage such as the <A HREF="http://www.coolermaster.com/products/hdd/hd.html" target="_new">DVD4002</A> or even better fit 3 HDs into 2 5.25" bays with a <A HREF="http://www.3ware.com/products/ata.asp" target="_new">drive cage</A>.