I've had a Samsung Focus for 2 years now. I had a screen protector on it for the first month or two, but it made the screen image look horrible and eventually started wearing off. I removed it and never looked back. In the 1 year and 10 months since, I've only received 1 tiny scratch on the top left of the screen, which isn't visible when the screen is on, nor is it easily visible with the screen off (you have to look at it from point blank distance to see it).
The back of the phone has always had something on it. Not a cover, just a decal (but it still prevented some scratches, nothing noticeable).
When I get my lumia 920 I'll never put anything on it. It's one of the nice things about modern smartphones, at least windows phones. No need to buy a cover to protect it or to change the color to what you want (*cough* iPhone *cough*), which helps keep the phone light and thin.