Yes, she is right -- and I hope you'll take steps to remedy this, even if you think your cheating will never be discovered. The Bible is clear: "Give everyone what you owe him: If you owe taxes, pay taxes" (Romans 13:7).
Think of it this way. If you owed someone a sum of money, would you try to cheat them by not paying it back in full? I hope not, because if you did that, in reality you'd be stealing from them. But we all know that stealing is wrong, whether it involves breaking into someone's house and taking something that doesn't belong to us, or simply failing to give them the money we owe them. One of the Ten Commandments bluntly states, "You shall not steal" (Exodus 20:15).
Is cheating the government out of tax money we owe any different? Not really -- because if we fail to pay what we legitimately owe in taxes, someone else is going to have to make up the difference. In other words, when we cheat on our taxes, in reality we're cheating our fellow citizens. We have a responsibility to support good government, not only by our votes but also by our taxes.
On one occasion, Jesus was asked if it was right to pay taxes to the Romans -- a secular, undemocratic government that at times could be oppressive and cruel. Let His reply guide your life: "Give to Caesar what is Caesar's and to God what is God's" (Mark 12:17).