These negative values were long-term and short-term capital losses (form 8949 Box A and Box D)
The rounding made no difference in my case, but for someone with many capital loss values, the errors could add up (or perhaps most likely just average out to no difference in most cases).
It makes sense that the IRS would avoid complicating things in their pub 15 with too many details of how to round negative numbers, but TurboTax is a software program and it should use the arithmetically correct method.
Anyway, thanks for your good info on pub15, etc.
