I think you have to be really careless to scratch the nexus 7's screen.
but of course I see people on youtube videos all the time placing tablets on tables screen down (why!?!), standing them up and letting them fall flat, letting their cats with sharp claws paw at moving game objects, etc. I'd say most damage to tablet screens, are pure negligence.
I do however have some tiny nicks in the plastic border, which really isn't due to mishandling, but rather because it's just soft plastic and very easily damaged. my tablet never bumps or comes into contact with anything but my hands, and it's generally used on clothe or cushioned surfaces. so I can't really explain the nicks other than the plastic border is really weak.
I bet if most people look close enough, they'll notice these same tiny nicks, either now or eventually. I think a year from now, most people will notice this issue and the device will be known to be prone to that sort of wear and tear. but I don't really care about the border as long as the screen is scratch free.....and so far, it is.
I have however noticed, what I believe to be a manufacturing issue with the screen. i noticed the glass is kind of sunken over the screen area but raised over the bezel area, and the disparity between the borders and glass is uneven compared to the four corners. granted, by a very small hair thin margin, but I can tell it both by touch (sometimes) and visually when viewing it from the side at an extreme angle. but, the plastic border is always raised higher than glass, so it's not really an issue. but near the corners, it is almost virtually flush.
if you turn the device off and treat it like a mirror, look at the reflections you might notice along the borders that reflections are slightly warped in some areas at some angles.that indicates the screen is not 100% flat.