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 $19.73   6.347%   $20.9823   $20.98   $21.00   $21.00  
  Purchase #2 $61.82   8.579%   $67.1235   $67.12   $67.10   $67.10  
  Purchase #3 $158.99   7.530%   $170.9619   $170.96   $170.95   $171.00  
  Purchase #4 $16.64   3.492%   $17.2211   $17.22   $17.20   $17.20  
  Purchase #5 $212.67   10.354%   $234.6899   $234.69   $234.70   $234.70  
  Purchase #6 $112.48   4.816%   $117.897   $117.90   $117.90   $117.90  
  Purchase #7 $193.71   12.522%   $217.9664   $217.97   $217.95   $218.00  
  7 purchase totals = $776.04       $846.8421   $846.84   $846.80   $846.90