Strictly speaking, the IRS instructions call for sending in (i.e., mailing) your Form 1099-B, or an acceptable substitute, to the IRS, listing each of your individual trades for the tax year, where the taxpayer chooses the option of making just a summary entry on Form 8949 (which then "flows" onto Schedule D).
As a practical matter, however, and as a CPA myself, I've found it perfectly acceptable to file a tax return with just a single summary entry in the capital gains and losses section, as long as the brokerage firm or other financial institution is clearly identified on Form 8949, and as long as the net proceeds, gains, losses, etc. totals exactly match the summary shown on the last page of the actual 1099-B received by the taxpayer. Really, that is all the IRS will be looking for . . . not all of your individual trades, especially where there could be hundreds, or even thousands of them. Rather than adding up your individual trades, what the IRS really needs is your net gain or loss summary, as that is what your tax calculation is ultimately based upon.
Certainly, I would strongly recommend to anyone that they keep their complete 1099-B as a "back-up" or proof, in the unlikely event that the IRS ever asks to see it, for as long as the statue of limitations exists for that particular tax return. However, there is no real or actual need to mail this document (or another summary of your brokerage activity and trades) to the IRS -- unless, of course, you are ever asked to do so.
If your capital gains / losses figures on your tax return agree to the last dollar with what your brokerage firm reported to the IRS, then that's all the IRS computers are really checking to see here.
Thank you for asking this important question.
