Prism Users Guide - Chapter 5. Taxes
5.11 Tax Rounding

In Node Preferences > Taxes > General, you can configure Prism to round item tax amounts up or down. For example, the calculated item tax amount might be 19.9955. The merchant can use tax rounding to ensure this tax amount is rounded up to 20.00 (or down to 19.99). 
Tax Rounding is supported for Sales Tax, VAT and ICM. Tax Rounding is not supported for Sales Tax - Multi and VAT - Multi.
1.    Navigate to Admin Console > Node Preferences > Taxes > General.
2.    Select the Round Tax Amount checkbox.
3.    Select the rounding method to use: 

• Normal: Tax $ is rounded up or down, whichever is closer.
• Up: Tax $ is always rounded up.
• Down: Tax $ is always rounded down. 

4.    Save the changes.
Tax is always calculated to four decimals; however, the number of tax decimals displayed (item-level) follows the setting in Node Preferences > Language & Locale: No. of Decimals - Tax. In the U.S., No of Decimals - Tax is set to 2. If the calculated tax amount is $0.1200, no rounding is done because none is needed.
How Doc Qty affects Rounding

• When tax rounding is OFF, the individual item tax amounts are stored to a precision of four decimals and then summed. The summed amount is then rounded for display. For example, if an item has a tax amount of $0.0045 in data, this will be displayed as $0.00. However, at Doc Qty 100, the extended tax amount becomes $0.45, and this amount is reflected in the tax subtotal.
• When tax rounding is ON, the individual item tax amounts are rounded then summed for the tax subtotal. An item with a tax amount of $0.0045 in data will round down to $0.00. At a Doc Qty 100, the extended tax is $0.00.