The 'Gold Standard' of screen protectors are usually considered to be the 'Invisible Shield by Zagg'. They are pricey though...usually like $29-$39 depending on the tablet. They haven't put up on their site one for the Nexus 7 yet...but if we go by the price of their Kindle Fire one they start at $24.99 and go up depending on if you go screen only or full body and some other options. Frankly, I'm not sure if it's worth it at that price. Another decent one is 'ArmorSuit'
which is more reasonable at $14.95 which gives you lifetime replacements as well.
Of course you can get generic ones for much less all over eBay and Amazon but it's luck of the draw if they're any good.
As for if the Nexus 7 screen can be scratched? Yes...even Gorilla Glass technically 'can' be scratched so i'm sure this Corning 'Fit' Glass can be too. The glass is hardened so that it resists most scratches in normal circumstances but let's say you drop the thing or if you happen to have a sharp object hit the screen in just the right way...it can still scratch it. Now if you keep your tablet in a case normally a screen protector might not be necessary. If you plan on using it without a case however a screen protector might not be a bad idea. Also there are some screen films/protectors that are anti-glare in case you don't want that reflective screen so that's another use scenario.