Precision should be part of the spec for integrations. With the integer multiple of minimal unit, that makes it clear in the API what it is.

e.g. it doesn't make sense to support billing in sub-currency unit amounts just by allowing it in your API definition, as you're going to need to batch that until you get a billable amount which is larger than the fee for issuing a bill. Even for something like $100,000.1234, the bank doesn't let you do a transfer for 0.34c.

For cases where sub-currency unit billing is a thing, it should be agreed what the minimal unit is (e.g. advertising has largely standardised on millicents)