SalesOrder RoundingOffAdjustment OrderRow

We have a requirement to ensure that the order total is rounded to the nearest “whole” number amount in kronors.

Perhaps its a pretty basic question, but I am trying to do this by added a SalesOrderRow to the cart with OrderRowType.RoundingOffAdjustment, but its not immediately obvious how this gets committed to the Cart OrderRows.

What is the “correct” way to do this? Should this be done in the CartController at the moment of placing the order? Or can it be accomplished with a ValidationRule? How do I properly commit it to the Cart OrderRows?

Any tips/ideas appreciated!

Litium version: 8.19.0