The iP4 is my 4th iPhone and I've never scratched the screen on any of them since starting in 2007 and I've never put any protection on the display on any of them.
They go in my pocket, in my computer bag, on the dash in the car, etc.. I do try to make sure I don't have grit on my hands when using it and I don't have kids and have never let kids play with them.
The display is tough and can take almost anything in normal use. It's also best to use them naked for the best iPhone experience.