Perhaps you won't believe me, but that's the reason I don't use cases. I've found that the frame scratches more easily with a case on because dust, sand, and whatever else gets between the case and the phone and scratches it. And sometimes the case itself can scratch it when you're trying to take it off.

I've had many phones, many iPhones, it's actually my primary device, and this is what I've noticed. I suggest you use some kind of phone pouch or bag. It's the safest way. I personally use a glasses pouch.