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 $45.71   12.487%   $51.4178   $51.42   $51.40   $51.40  
  Purchase #2 $241.55   8.448%   $261.9561   $261.96   $261.95   $262.00  
  Purchase #3 $145.41   12.703%   $163.8814   $163.88   $163.90   $163.90  
  Purchase #4 $113.29   8.201%   $122.5809   $122.58   $122.60   $122.60  
  Purchase #5 $24.40   12.330%   $27.4085   $27.41   $27.40   $27.40  
  Purchase #6 $122.59   5.378%   $129.1829   $129.18   $129.20   $129.20  
  Purchase #7 $23.48   4.282%   $24.4854   $24.49   $24.50   $24.50  
  7 purchase totals = $716.43       $780.9130   $780.92   $780.95   $781.00