Not quite.  "Render unto Caesar the things that are Caesar's" doesn't stop at the coins with his pics on them.  All commerce in Caesar's domain is taxable by him.  This world is not as soft & fuzzy as you've been lead to believe.  Unfortunate fact number one:  The government is not stupid.  Unfortunate fact number two:  You are not the first one to think of taking a country's currency out of equation to avoid taxation.  Honest.  There were others.  Sad conclusion:  The clever tax loophole you think you've discovered has been successfully addressed & plugged up.  At your expense.  

No matter how slow & unwieldy government bureaucracies are, a few millenia is usually enough.  Though this is unsettling, governments put plenty of thought into parting you with your delicious caek, and have been for quite a while.  That's pretty much what they do, that's how they got to God Mode.  No noob is going to pwn them by "not converting into fiat."  If you ever wondered why major corporations don't avoid taxation by trading with bricks of gold instead of gobment money, and why you have to pay taxes on your house (where the only fiat involved is the stuff you need to fork over), there's your answer.