It depends of how careful you are with it and the case you have when you carry it around, and everything. I personally don't have one but I have a smart cover, as well as a full back and front cover for transportation. But, I would say that having a screen protector on every devices is a good thing. It will prevent your phone from unwanted scratches.
I have an iPhone 4, a 4S, and an iPad and have never put screen protectors on them. None of them have scratches, yet. I don't want to dull the dynamics of the crystal clear retina displays with a piece of plastic film.
Sent from my iPhone 4S or iPad 3rd gen, whichever I happened to have in my hands at the time, using iPF.net
I always put one on any device I have, but I make sure to have a nice quality one so I don't ruin the look of the screen. Go with the "Steinheil Clear Ultra Crystal Screen Protector" your screen will look just as good as a retina screen with no protector.