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 $19.36   8.579%   $21.0209   $21.02   $21.00   $21.00  
  Purchase #2 $17.65   11.749%   $19.7237   $19.72   $19.70   $19.70  
  Purchase #3 $140.92   5.603%   $148.8157   $148.82   $148.80   $148.80  
  Purchase #4 $213.46   4.765%   $223.6314   $223.63   $223.65   $223.60  
  Purchase #5 $41.30   13.370%   $46.8218   $46.82   $46.80   $46.80  
  Purchase #6 $56.36   4.171%   $58.7108   $58.71   $58.70   $58.70  
  Purchase #7 $134.76   12.507%   $151.6144   $151.61   $151.60   $151.60  
  7 purchase totals = $623.81       $670.3387   $670.33   $670.25   $670.20