If you carry anything metal in the same pocket as your iPhone, it is only a matter of time before you scratch the screen.
I have carried my iPhone 5 for the past couple years without a screen protector. I have tried to be careful about keeping it in a pocket by itself, but have slipped up a few times. I have a couple of scratches, but truth-to-tell, it really doesn't bug me. When the screen is on, you can't see the scratches. So it would only be readily apparent if you are the type of person who likes to stare at an unlit iPhone screen.
Only you can answer the question of will the scratches bother you.
That being said, screen protectors tend to pick up scratches way easier than the uncovered screen. So look at your iPhone with a screen protector. How many scratches do you have? Does it bug you? I suspect without the screen protector, you would probably have 1/4 of the number of scratches.
You could try one of the tempered glass screen protectors now being sold (with which I have no experience) which will be more scratch resistant, and may be less objectionable to you than the typical plastic screen protectors:
http://www.spigen.com/iphone-5-screen-protector-glas-t-slim-premium-tempered-glass.html