My opinion - you're asking for trouble by NOT using a screen protector.
It just takes one drop--even with a good case--for the screen to scratch, crack, or shatter.
Use something like Skinomi or InvisibleShield, and your phone's screen will not scratch, crack, or shatter. When you go to resell the phone, it'll be in great shape.
Just to show you how much of a life saver screen protectors/cases are, check out my attached picture. My iPhone 5S was BENT by accidentally slamming it in a car door not once, but twice (I stupidly thought my seat belt was caught in the door, closed it again and discovered the iPhone). Without my case and Skinomi, the 5S would have broken. With the case and protector, the 5S survived with no scratches and the screen in tact. So much so that the Apple store gave me a new 5S because they just assumed the body warped over time.