I think two things are conflated in the question: helping others for no money or only a token gesture of thanks (eg helping a friend paint their house for a beer), and doing a side job for cash (eg house painting for acquaintances on the weekend for cheap rates).

I'm pretty sure the first is totally fine in any country. Also pretty sure the second would be considered a tax-dodge in any country as it's just income you don't declare. In Ireland those doing jobs for cash like that is called a "nixer".