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 $20.37   3.427%   $21.0681   $21.07   $21.05   $21.10  
  Purchase #2 $28.86   11.603%   $32.2086   $32.21   $32.20   $32.20  
  Purchase #3 $116.44   6.326%   $123.806   $123.81   $123.80   $123.80  
  Purchase #4 $70.37   6.167%   $74.7097   $74.71   $74.70   $74.70  
  Purchase #5 $170.88   11.627%   $190.7482   $190.75   $190.75   $190.70  
  Purchase #6 $187.87   9.709%   $206.1103   $206.11   $206.10   $206.10  
  Purchase #7 $194.36   13.763%   $221.1098   $221.11   $221.10   $221.10  
  7 purchase totals = $789.15       $869.7607   $869.77   $869.70   $869.70