One good way to avoid getting a notice from the IRS is to report all your income in the first place. That way when they match the W-2s and 1099 they receive from payers, the IRS will not be sending you a CP2000 notice. Our tax code specifically requires us to report all world wide income. Now, there is income that is not taxable, but what I am talking about are clients who choose not to give me information and get caught later when the IRS matched everything. Yes, some of them are people who are trying for the best current refund and think they can get by with "forgetting" a W-2 or 1099. But, the rationals for not reporting W-2 or 1099 income I see most are the - "I didn't get anything from (the payer)," "it's not very much" or "the taxes have already been taken out."
1099's don't have to be issued unless the taxpayer has received a certain amount if money. It depends on the specific 1099. But even if you don't get a 1099 (or W-2G) that income is reportable. And the fact you haven't, yet, received a form does not mean that the income can be omitted. While this income may not be caught in the matching process, it does still have to be reported.
My issue with the "it's not very much" group is their definition of "very much." I have had early filers, who are still missing a W-2, try to tell me that it won't be much since they just worked there for a few days. When they bring me that W-2 so that we can finish that return, it is for hundreds or thousands of dollars. What many people don't realize is that sometimes as little as $100 added income can change a refund much more than the tax on that income due to its effect on credits and phaseouts.
Just because taxes have been withheld doesn't mean that the withholding is enough to cover the taxes on that income. This is a mistake a lot of people make. W-2 withholding is based on the W-4 you completed and gave to your employer. That might or might not be enough to cover the tax and most people understand that concept. But when they take a distribution from a 401K or IRA, they assume that withholding covers everything and no more tax is due on that money. The problem is generally these are set percentages or amounts that do not take into consideration your specific circumstances. Two taxpayers taking a $10,000 distribution from their IRAs may both find that 20%, $2000, federal withholding is taken. For one, that may be more than enough and they will get some money back in a refund. But the other will have to pay a balance due because the 20% doesn't cover the tax at their tax rate. This is also getting to be a problem with some W-2G payers.
The goal should be to report all your income on the original return. And most taxpayers try to to do that but they are operating under misconception or bad information. The problem with not checking with a professional is that the IRS does match the returns to the income they have reported for you. When these don't match, they will send you a notice and if they are right you will pay more than if that income had been on the original return.