opinions vary -- and my own tastes and prescriptions likely differ from others. my personal preference is to stamp on the front right top panel -- this avoids most of the security features and gives less of a reason for banks to remove the note from circulation. plus, I just find it distasteful to stamp on anyone's face. banknotes are a national icon and I try to not detract from them. on the back, my stamp fits perfectly on the opposite side of the foil security strip. this gives a backing to the stamp and if that is the ONLY place it is stamped, the note still works in many vlts and slot machines. again... your results and preferences may be different from mine. my suggestion is to familiarize yourself with all the security features and try to avoid rendering them less effective and then let common sense guide you.