Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $21.85   5.695%   $23.0944   $23.09   $23.10   $23.10  
  Purchase #2 $9.44   11.422%   $10.5182   $10.52   $10.50   $10.50  
  Purchase #3 $188.31   7.391%   $202.228   $202.23   $202.25   $202.20  
  Purchase #4 $107.36   13.871%   $122.2519   $122.25   $122.25   $122.30  
  Purchase #5 $14.36   4.475%   $15.0026   $15.00   $15.00   $15.00  
  Purchase #6 $234.37   7.168%   $251.1696   $251.17   $251.15   $251.20  
  Purchase #7 $59.88   12.454%   $67.3375   $67.34   $67.35   $67.30  
  7 purchase totals = $635.57       $691.6022   $691.60   $691.60   $691.60