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 $21.26   3.200%   $21.9403   $21.94   $21.95   $21.90  
  Purchase #2 $15.77   8.698%   $17.1417   $17.14   $17.15   $17.10  
  Purchase #3 $35.37   3.775%   $36.7052   $36.71   $36.70   $36.70  
  Purchase #4 $244.55   12.140%   $274.2384   $274.24   $274.25   $274.20  
  Purchase #5 $27.26   5.872%   $28.8607   $28.86   $28.85   $28.90  
  Purchase #6 $8.62   5.575%   $9.1006   $9.10   $9.10   $9.10  
  Purchase #7 $87.39   3.185%   $90.1734   $90.17   $90.15   $90.20  
  7 purchase totals = $440.22       $478.1603   $478.16   $478.15   $478.10