If you do not want to mail stuff to the IRS, you have to enter some data manually. To see what you have to enter you should run a dummy return, where you do not enter any data; only download 1099-B(s) from brokerage(s).
Then go to Forms and look at (and print) Sched D and the 8949s the program generated. The 8949s will only show the transactions that have adjustments (Wash sales, A or D will be checked), and transactions where the cost is not reported to the IRS (B or E will be checked). The other transactions are lumped on line 1a and line 8a of Schedule D. The IRS does not require that the transactions on line 1a and line 8a are transmitted with e-filing and that is why you do not have to enter them individually: You just report the total of these transactions. All the other transactions have to be entered manually (All these entered amounts will be rounded), but most likely there will not be that many transactions to enter. To get the not-rounded total for short-term transactions, add the non-rounded proceeds and cost of the short-term items that are not on line 1a and subtract them from the total proceeds and cost for the short-term transactions. Do the same for long-term transactions. Now start the real return and arriving at the 1099-B, use the step-by-step mode, with "I type it myself". First use "I'll enter a summary for each sales category" and enter the 1a total proceeds and cost (Category A). Do the same for the 8a items (Category D). To enter the other transactions, choose "I'll enter one sale at the time" with the right category and enter each transaction as shown on the dummy 8949.
I used this method and it reduced the rounding error from $76 to $2. For the Box B and Box E category, you can choose to round up or round down the cost (which is not reported to the IRS) to achieve an even more accurate total for the gain.
