Mine suffered a few dings on the casing but this was my fault and not anything related to the painted aluminum wearing off.
You can go either way. I just put an Otterbox Defender on mine.
Side Note: Scratches tend to be more visible on black devices versus white. This includes the rear aluminum casing.
I've never used a case with any iPhone as all that I've tried I just hate using the iPhone in them.
I do carry mine in a sleeve (I like Sena's leather sleeves) so when it is in my pocket it is protected quite well. But I can slip it out of the sleeve and use it naked, the way I like.
My iPhone 3Gs, iPhone 4 still looked like new after about 2-3 years of use each, and my iPhone 5 is the same after not quite a year of use (my iPhone is my only phone).
As mentioned, it really is a matter of personal choice based on your likes and how and where you use your device.
I never used a case before because I loved my phone being really slim. But all it takes is one drop and you will def get a scratch dent or shattered screen. I now use a pretty slim ZooGue case but can drop it 4 feet or so without a problem as I have done several times.
They are having a sale right now where you can get 1 for free just pay $3.99 Shipping or 2 for $5.99 http://www.zoogue.com/iphone-5-social-pro-cases/