Here's a direct copy and paste from IRS Publication 17, under computations section:
Rounding off dollars. You can round off cents to whole dollars on your return and schedules. If you do round to whole dollars, you must round all amounts. To round, drop amounts under 50 cents and increase amounts from 50 to 99 cents to the next dollar. For example, $1.39 becomes $1 and $2.50 becomes $3.
If you have to add two or more amounts to figure the amount to enter on a line, include cents when adding the amounts and round off only the total.
Please correct me if I read it wrong:
1. Rounding is optional not mandatory.
2. If you do rounding, only rounding the end results but not each amount needed to compute that result.
I believe TT is rounding each cost basis and proceed first which introduce even bigger errors. Also, why not give your customer the choice of enabling rounding or not in the first place?
