I am still using my iBook (buying PB in Jan), and I still have the original screen cover that came with the machine two years ago. I put it into the machine whenever I close the lid.
My best friend has a 17" (rev a). He found that he DID need to do this. His dad has a rev a 17" also, and doesn't need to do this.
...I guess just do it as a precaution until you get a feel for your particular machine and whetheer or not it is required.