Prism Users Guide - Chapter 5. Taxes
5.11 Tax Rounding

Tax Rounding
In Node Preferences > Taxes > General, you can enable tax rounding and select a rounding method. If selected, the tax amount (Tax $) is rounded at the subtotal level (rather than rounding each item's sales tax or order tax amount). Important! You also need to set the number of tax decimals to zero in Node Preferences > Language & Locale.