How many HDD do you have? If you have like one or two, you could mount those in the two 5 1/4" bays under your CDROM drives. Then you could either leave the bay covers off OR drill some nice looking holes in them and have the HDD get plenty of air from your cooling system inside.
BUT that is just me
Personally, I would do one more fan in the front and one more in the back. Doing all of them, IMHO, is just overkill and creating a noise generator. Your call though.....