In my opinion, its all about personal preference.
I prefer to leave the iPod as it is, the screen is meant to be scratch resistant, however after a few weeks of leaving the iPod protector free, i could notice a tiny scratch on the screen however it was impossible to see when the screen is turned on.
If you are very OCD about getting scratches, then yes, a front and a back cover is needed, but i think the main accessory you need is a case to protect the back, as the aluminum back does make the iPod look quite good if it is perfect.