Keep in mind that if they're not earning much, they're probably not paying any income tax at all, and until they're earning at least $38,700, the most they'll be taxed on their taxable income is 12%.
I don't think that supporting tax evasion should be a primary reason for tipping in cash.
But there are other reasons why tipping in cash is a good idea, such as making sure that the money actually goes to the worker, and knowing that they'll have immediate access to it, rather than having to wait until their next paycheck.
