MacBook keyboard burning into screen

Hi.
With the LED glossy screen (which I believe is made of glass) on the new MacBooks, does one need to be concerned about the keyboard getting 'burned' into the screen when the lid is closed?
A lot of older laptops that I have used or have seen have the keyboard pattern burned into the screens.

I know the new MacBooks have a different type of screen. I also think that the keyboard burn is from packing the macbook or laptop into a bag that is too small for it. My questions are basically:

Do I need to worry about my keyboard burning itself into my (glass?) screen on the new macbook?
If this were to happen, would apple replace it for free?

I have the thin protective sheet that came in the box when I bought the MacBook and I have yet to close the cover without that sheet between my keyboard and screen. Do I need to do this?

Thanks in advance.