I get your 'why' David, but from an audit point of view, I think you are overthinking your worries.
From a cost control point of view, that is easy to manage by setting yourself a budget and sticking to it. You can always get a credit card that has no FX transaction fees allowing you to always pay in local currency at better rates with no charges. Failing that, you could always pre-load a travel card with a set budget (like an electronic version of the old travellers cheques) to ensure you stick to plan. Whatever the case, cash is now almost irrelevant.
That said, I always take a small cash amount for tips, emergencies, markets etc when abroad.