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 $19.40   9.439%   $21.2312   $21.23   $21.25   $21.20  
  Purchase #2 $124.93   5.821%   $132.2022   $132.20   $132.20   $132.20  
  Purchase #3 $237.83   9.797%   $261.1302   $261.13   $261.15   $261.10  
  Purchase #4 $27.47   4.721%   $28.7669   $28.77   $28.75   $28.80  
  Purchase #5 $70.39   11.248%   $78.3075   $78.31   $78.30   $78.30  
  Purchase #6 $17.55   7.742%   $18.9087   $18.91   $18.90   $18.90  
  Purchase #7 $207.85   10.580%   $229.8405   $229.84   $229.85   $229.80  
  7 purchase totals = $705.42       $770.3872   $770.39   $770.40   $770.30