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 $19.58   9.223%   $21.3859   $21.39   $21.40   $21.40  
  Purchase #2 $249.56   5.785%   $263.997   $264.00   $264.00   $264.00  
  Purchase #3 $13.31   4.481%   $13.9064   $13.91   $13.90   $13.90  
  Purchase #4 $49.44   5.400%   $52.1098   $52.11   $52.10   $52.10  
  Purchase #5 $133.94   8.419%   $145.2164   $145.22   $145.20   $145.20  
  Purchase #6 $107.34   8.750%   $116.7323   $116.73   $116.75   $116.70  
  Purchase #7 $28.84   13.728%   $32.7992   $32.80   $32.80   $32.80  
  7 purchase totals = $602.01       $646.1470   $646.16   $646.15   $646.10