If you received a W-2 or a 1099 in the mail, the government got one too, so you’re not fooling anyone if you don’t acknowledge it at tax time. Companies are required to send statements to anyone they paid more than $600 in the year – if you earned that much from a job and haven’t received one, make sure you do. If someone paid you less than $600, and didn’t give you a 1099, you’re still responsible for reporting all your income. Yes, cash and all.
In the eyes of the IRS, not declaring income because it's 'under the table' is a form of tax evasion. If you don’t report 25% or more of your income then you’re committing tax evasion. That’s right, a felony. Not cool.
“Under the table” sounds cute and friendly, but “tax evasion” does not sound like something you want to be associated with. These are just two ways of saying the same thing.