i personally use a scanner. i find you get a lot more details and clarity, especially when it comes to holograms and such. i scan all of my images at 300 dpi, then reduce them into more manageable sizes for further tweaking/sharpening.
the only downside is that newer scanners/software come with anti-forgery technology that won't allow them to work with images of banknotes. some of them check for certain shade combinations (not too hard with u.s. currency), and others for hidden circle patterns (the musical notes on uk pounds, some of the yellow 0's on the back of the 2004 $20's, etc).
if you have an older scanner and older software that works, you'll find plenty of advantages with it over using a digi-cam.
http://public.fotki.com/sanmiguel/world_banknotes