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 $20.39   3.105%   $21.0231   $21.02   $21.00   $21.00  
  Purchase #2 $36.37   3.796%   $37.7506   $37.75   $37.75   $37.80  
  Purchase #3 $184.68   3.375%   $190.913   $190.91   $190.90   $190.90  
  Purchase #4 $64.34   10.131%   $70.8583   $70.86   $70.85   $70.90  
  Purchase #5 $73.62   10.688%   $81.4885   $81.49   $81.50   $81.50  
  Purchase #6 $200.46   3.575%   $207.6264   $207.63   $207.65   $207.60  
  Purchase #7 $100.27   10.432%   $110.7302   $110.73   $110.75   $110.70  
  7 purchase totals = $680.13       $720.3901   $720.39   $720.40   $720.40