Penny Rounding Rules are used to determine how and when to round cash-only Payment amounts.
These Rules are maintained at Accounting / Setup / A/R / Penny Rounding Rules. Entering an amount in the Dollars field and selecting Test provides the rounded result which is the amount the Customer would owe.

Once Rules are set up here, they can be set on the Location's G/L Posting tab and will be the default Rule used for Payments on Accounts and Quick Tickets (for cash-only Payments).