The software typically starts the rounding early in the computations, but you likely only rounded your final result. For reference, "standard" tax rounding, amounts should be rounded to the nearest whole dollar by dropping amounts under 50 cents and increasing amounts from 50 to 99 cents to the next dollar. For example, $2.30 becomes $2 and $2.50 becomes $3. I don't think a difference under a dollar or two will be an issue. It never has with payroll, which has rounding drift (not rounding errors) every pay cycle.