I'm sponsoring my wife and stepchild for green cards in the U.S., and now have to turn in my Affidavit of Support on Form I-864. In reading the instructions, it seems like I have a lot of options for providing the federal tax information: such as submitting one, two, or three years' returns, and submitting them either in the form of a copy of what I sent the IRS or requesting an IRS tax transcript. What's best? Does it matter?
Although you do have options in the case of supplying financial documentation, you're right that some options may be more convincing than others.
Regarding your first question, as you noted, USCIS gives you the choice of submitting only one year's tax information, or submitting up to three years' worth. You'll want to make a strategic decision here, based on your income history over the last three years.
If, for example, you've been promoted at your job for the last three years, so that your current year's income is higher than the previous two years', there's no reason to submit anything more than last year's tax returns.
If, by contrast, you had a difficult year last year; perhaps were out of work due to an illness or furlough; but the previous two years' tax returns show a more "normal" (and higher) income stream for you, it would be wise to submit those as well.
As to the form of submitting this information, copies of your own tax returns are acceptable, but the immigration officials know that it wouldn't take much for someone to forge these.
Therefore, they prefer to receive tax transcripts, which are generated by the IRS and show detailed information of your tax filings. The IRS offers a free online portal at which you can download and print your tax transcript immediately. Go to its "Get Transcript." page.