What currency to use in unit tests?

Use USD (or, generally, whatever currency is most commonly used in your application). I say this for two reasons:

• Good test data is unremarkable in every respect except that part of it which the test is actually about. When you're writing tests that don't have anything to do with differences between currencies, you don't want to have to think about differences between currencies. Just use whatever is most natural in the application.

• The idea that using an unusual currency everywhere will somehow result in better testing of unusual currencies is a red herring. Tests should be explicit and focused. If you need to test something about a specific currency, write a test whose point is to test that thing. And if a test is not about a specific currency, it shouldn't break when handling of some unusual aspect of that currency breaks -- it's not valuable to have half your tests break for the same reason; you only want one to break. So there is just no need to spread unusual currencies around the test suite and hope that that will catch something. Instead, optimize for readability; see point 1.