When advertising bug 24158 we had some discussion about further improvements to currency handling in Koha. It would be nice if the currency rate used for the conversion on ordering and receiving would be shown in the form (pulled from administration > currencies) but be editable at the same time, so you can update it if necessary. We could also show a checkbox to update the currency rate in administration from there, visibility of the checkbox depending on the existing manage_currencies permission. This could go together with bug 25655 that suggests storing original prices and rates used.