If you have a workflow dealing with the entities that include money fields, avoid setting explicit currency. Instead, try to infer the currency from the related records.
For mere mortals, it’s just a bad practice, for ISVs, it’s definitely a case of spießrutenlaufen.
Why? Because if you activate your workflow and add it to a solution, then importing your solution to any CRM organization that does not use MMDDYYYY as their date format (and therefore will opt for their own currency), will complete with the warnings and the workflow on the target system will be deactivated.