Is the worker obliged to abide by the hours stipulated in the contract only if the employer evades taxes and does not declare all the actual hours and wages?

It is understood from the question that employers write employment contracts with fewer hours and lower wages than the reality to evade taxes. There is no blame upon the worker to work more hours than what is written in the contract and receive wages for them, as long as he has agreed with the employer on the real hours and wage, and the responsibility for taxes falls upon the employer.