The current rounding as implemented works perfectly, as nothing is broke there is nothing to fix.
Rounding shouldn’t take into account the taxes as rounding is a financial adjustment to the Invoice’s total, it’s not an adjustment on the transactions making up the total. Taxes should remain true to the line item price.
How would you proportionally adjust the taxes if the invoice had both taxable and non-taxable items and the invoice total rounding was only small, say 0.09. If the taxable items added up to be 60% of the total, are you going to attempt to adjust the taxes for 60% of the 0.09.