By and large, the government has better things to do than go after the penny players.
However, financial institutions are required to report suspicious transactions. What is regarded as suspicious depends upon the financial institution and the client.
E.g. a self-employed builder making regular mortgage payments in cash would be suspicious, and the mortgage bank may notify the authorities.
If you suddenly start making regular cash deposits of the same amount on a very regular schedule than that also may be suspicious.
Worst comes to the worst, your bank will just ask where the cash is coming from. My stockbroker screwed up once, sold all my stock and closed my account, and wired the cash to my checking account. For whatever reason, the transaction just came up on the statement as 'bank transfer <Insert name of bank that stockbroker is a part of> Bank'. I got a phone call from my bank manager saying 'WTF is going on? Where's this money come from?'