There is no such thing as a perfect tax return, so no software package can prepare such.
The tax laws are not black and white if you have any sort of complexity to your investments.
Every year Money magazine has 50 tax preparers work on the indentical information and gets back 50 different answers. And most of those tax preparers use software much more expensive than TurboTax, TaxCut, etc.
So do I. I file for an LLC, an individual, and a partnership. I have still never been audited, perfect return or not. They are not that difficult. Call any IRS agent, CPA or tax attorney and you will get different answers all the time. The system is based on trust. While many people cheat, using all the legal loopholes and advantages will not cause a defective return.