With Corning Gorilla Glass it is probably superfluous and nothing detracts more than screen protector with any trapped air bubbles!
Symbian specialist until 31/01/2013 with interests in Maemo & MeeGo but an aversion to Windows!
Corning Gorilla glass is scratch resistant and very tough. It is however not scratch proof. Using a screen protector is a personal choice and would never hurt if you choose to do so. Make sure though you use a protector which will leave the proximity sensor free and functioning.
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Gorilla glass is scratch resistant and not scratch proof. It does not scratch as easily as normal glass but it will still scratch under the right circumstances. Personally I have yet to see a completely scratch free Gorilla glass after a few months normal use.