Never. I can’t stand cases or screen protectors.
Just get AppleCare if you’re worried about it. Then use the phone the way it was built to be used, in all of its sleek and beautiful glory.
I used to agree. Right up until the current design (iPhone 6). The phones are too thin and the materials too smooth. Plus the round edges complete the bar of soap handling Apple is apparently going for. They slip out of the hand so easily that they are not comfortable to use without a case. Plus the bump on the X means it won't sit flat on a table without a screen. To be honest, it is almost like they have designed them with the intention of people using a case.
I have used the Apple leather cases since the 6. They are fantastic and in my opinion, make the phone more usable.