RCW 82.08.054

Sellers shall compute the tax due under this chapter and chapters 82.12 and 82.14 RCW by carrying the computation to the third decimal place and rounding to a whole cent using a method that rounds up to the next cent whenever the third decimal place is greater than four. Sellers may elect to compute the tax due on a transaction on an item or an invoice basis. This rounding rule shall be applied to the aggregated state and local taxes.