To be able to use smart rounding for prices that start on the third decimal the smart rounding rules needs to be set up with the ‘price quantity’ field for ‘unit price from’ and ‘unit price to’.

Example:

Unit price = 4,5 / 1000 pcs (0,0045/pcs)

To use smart rounding for this price a price quantity will have to be set for 'unit price from' and 'unit price to' in the smart rounding rules.