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.

Anyway, the allocation may be tweaked to favour where the additional/missing pennies will be allocated. You can find more at: https://github.com/eriksencosta/money/blob/trunk/docs/usage/...