[mikeash]

The work is already being done. Try putting some incorrect numbers into your tax return this year. Sometime after you file, you'll probably get a letter informing you of the mistake, telling you how much your actual tax liability differs from what you filed, and either a check or a bill for the difference.

[gpawl]

IRS checks your math and makes sure all the reports (your copy vs employer/bank/vendor copy) match. They won't tell you which deductions you are supposed to take.

[sedachv]

> They won't tell you which deductions you are supposed to take.

They will for the most basic personal deductions. I have gotten a correction on a 1040EZ where my deduction was corrected upwards and the refund increased. It is impossible for the IRS or your tax preparer to know things like what kind of automobile deductions you can take for your sole-proprietorship business. This is an impossible problem to solve - every change to the deductions allowed is going to make some people pay taxes on what should be business expenses/losses and/or allow other people to claim legal-but-bogus or just plain fraudulent deductions.

[maxxxxx]

That's exactly it. The IRS would just tell you what they already know.