I've used my 4S and 5 au natural, no case or protection at all, and neither have had significant scratches. My phone goes in my front left pants pocket solo, keys, change, lighters, etc. go in the right. I've done this since I had my 3GS so it's ingrained habit now.
When at my desk I place the phone on a notebook or something similar. I just avoid placing it directly on a hard or textured surface, and it's all good.
My 4S was pristine at sale (i5 launch), and my 5 has a couple small scratches from sliding outta my pocket & under my car seat, but that's it. All in all I'm happy w/how my phones have lasted & aged all while being able to appreciate Apple's design, unencumbered by cases, bumpers or anything else.