We discussed that on the iDempiere Chateau Chapelle Workshop 2019.
We like to set a currency rate per document because the rate may change during days or for other reasons. We like a field to set a "manual" currency rate calculation in a document header.
There is a patch from TrekGlobal that uses a spot currency rate type (e.g. "manual") that allows to enter a currency rate for every single document. Carlos likes to review the TrekGlobal implementation.