Yes, definitely always include the number of digits, but at your system boundary you still have to pray that whoever you're working with isn't silently dropping that number and re-deriving it from their own, almost-4217-compliant currency database.

Redundancy can be great, but it's not a panacea, since it's not guaranteed to be used in an optimal way.

ISO 4217 also defines the number of significant digits after the decimal separator if Wikipedia is to be believed.