> The IRS already prepares your taxes for you. There is simply no way around this: they have to prepare them to check the numbers they get from personal filings

I'd be interested to hear that I'm wrong, but I believe this is not true. The IRS almost certainly does not calculate on their end what your taxes should be, but rather uses some much cheaper heuristic which they compare to your return and if it's "close enough" then the amount they stand to recover from auditing you is less than the cost to audit, and the IRS simply accepts the difference.