I don't know whether trying to avoid taxes in your country is common and considered the right action. But I know it's against the law because the government won't like this, so once we intend to do things that may be illegal, it's best not to let a second person know. Because there is no guarantee that they will keep your secret. Not to mention whether bitcoin is legal in your country or not, because if it is illegal then your crime is even worse.
By the definition, tax avoidance is legal while tax evasion is illegal, but both of them are using a loopholes from the tax laws.
Imagine if someone make $10 Million per year by freelancing which make he get taxed on the highest income tax bracket i.e. 37%. He want to declare his income as a corporate income which make him need to pay only for 21%.
Here's the illustration:
Bob (millionaire): Hello Mr. Jack, I want to declare my personal income as corporate income, how to do this?
Jack (accountant): Mr. Bob, why you want to do that? I will report you to tax officer!