I think it's a terrible idea to do your return by hand.
Thought experiment: Is there anyone who actually does tax returns for a living who's decided to "go manual"? Gosh, no way! You dramatically lower your tax return's quality by doing it by hand.
P.S. I should say that we have to do some bits and pieces of really complicated tax returns by hand when the software doesn't do the work for you. That always ends up being a bad situation... risky, prone to errors, ugh.
First, a CPA would probably say that. Understandable.
Second, you are absolutely right, a professional taxpayer would use software. But I would hope they are familiar with the tax code before they use the software, otherwise they're a trained monkey.
Third, in reference to my original post, it adds time, but the value one gains is in understanding the tax code and how to apply it to their personal life (this is the MMM forums, where DIY isn't typically criticized but rather praised). I would also recommend one looks at a previous year's return if doing their own return by hand for the first time.
Fourth, what do you even mean by "lower your tax return's quality by doing it by hand"? This doesn't even make sense, unless you think people are missing valuable tax breaks or making addition or subtraction mistakes. The IRS doesn't give bonus dollars for a "quality" tax return.
As to your postscript, if things get really complicated you should hit up a CPA (as MMM decided to do once he hit it rich with business income).
Okay, first I want to apologize for saying this is a terrible idea. That was too close to an ad hominem attack which forum rules and also good manners prohibit.
But to address your points, first, doing the calculations by hand as
@seattlecyclone notes leads to arithmetic errors in your return. Even a simple return includes potentially dozens and often hundreds of calculations. Doing the return manually means errors. Those errors mean all sorts of bad things including an increase in the risk of an audit. Further, unless the mistakes increase your taxes, you're almost guaranteed to need to follow-up and correct your errors when IRS sends a CP notice.
Second, if you've got your numbers into the tax software, you can usually easily do some what-if analysis simply by changing a number or two. Note that you can't with tax law just use "rules of thumb"... you usually want to run the calculations. E.g., are you sure you want to use the standard deduction? Yes, it seems like it gives you a better number than your total itemized deductions... but what about the effect on your AMT? You could redo the calculations again by hand... or you can check a box and let the software do the math.
Third, the software should include hundreds (sometimes hundreds) of diagnostics that look at your calculated results, at the fields you've filled, at the fields you left blank, and then point to possible errors or opportunities. Even if you know the forms and the instructions really well, a human isn't going to do a good job at building her or his own list of diagnostic checks and then remembering to "run" those diagnostics every time an input changes.
