The thing is, it doesn’t really help. A system I’m working on can have hundreds of monetary amounts in the same currency. Storing the currency against each one is an awful waste of space.
Equally the allocation trick is good, but you’ll find that the people who really care about this stuff really care about the exact algorithm you’re using and who it favours.
I think it helps to have the inputs and outputs of the system fully representing a "money" (i.e., a monetary amount + its currency code in places like API resources, events, and so on). This way you can internationalize or support multi-currency processing if that's the case. That's (if I remember correctly) what the payment card services companies (e.g., Mastercard, Visa) do by default in their message exchanges when calling the issuer for transaction confirmation.
Equally the allocation trick is good, but you’ll find that the people who really care about this stuff really care about the exact algorithm you’re using and who it favours.