If we look at this a different way, the style sheet may make it look different but the content is the same, so you don't really need two of the same content indexed. This only affects the robots so a user wouldn't see a difference and could still access the printer friendly version, presumably you would have a link to the printer version on the original and so that's why its good for the original.
So to clarify - If you have the same content across multiple pages you want to select one page as the original and link back to that page via the tag in all of the duplicates.
I hope that helps clear it up for you.
