Hi,
there are some issues with add. currency residuals handling, unfortunately. Additionally, NAV2013 has an issue with add. currency on customer/vendor application and different exchange rates. It has been fixed in the Update Rollup 1 (Build 34587). Here is a link: http://bit.ly/16cchGn I haven't found it in the hotfix list (it is quite long, though), but the code pieces in CU12 showed up in Mercurial.
with best regards
Jens