Despite what the documentation says, it seems the default options in rails 3.2.6 allow you to just make things work with even less path information in your CSS.
E.g. ../app/assets/images/rails.png is references in your example.css.scss file with something like:
background: white url(rails.png) repeat-y;
You don’t include the image-url or asset-url into your scss (as far as I know), just plain url(your_image.png). That bit of documentation appears to be just an explanation of what it is doing in the background.