On the contrary, I feel that currency (or no currency) is a very interesting part of nation-birth that I feel should be investigated and discovered properly.
YOU are in the control of natural resources and such, and you already have control over currencies that way.
Besides, in medieval times there were no real "exchange" rates.
EDIT:
And I feel that issuing a global currency is just too unrealistic. Where's the backing? Why should people use a strange money they know nothing about? Where do you exchange it for gold? There are zillions and gillions of problems with this scenario, mostly because medieval tech-level may or may not have even traditional metal currency!
