****- Vibrations (for a high res scan) can kill it. Now, your "stupid question" isn't so stupid because I have no idea. I've never changed the dpi setting/level after I've captured the image. It's also possible (with modern banknotes) that less is more - all the overlapping colored lines are likely part of the anti-counterfeiting measures so the more detail you try to capture the less sharp it becomes (complete speculation).-    
 *****And a completely off-topic question: I'm going a few places, and one is a non-FOP country (Morocco). I will take all the pictures I can for myself. In terms of what I can use for Wikipedia, in practical (general) terms, what will not fly?-