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 $28.81   5.489%   $30.3914   $30.39   $30.40   $30.40  
  Purchase #2 $70.41   11.502%   $78.5086   $78.51   $78.50   $78.50  
  Purchase #3 $125.93   4.519%   $131.6208   $131.62   $131.60   $131.60  
  Purchase #4 $18.77   5.263%   $19.7579   $19.76   $19.75   $19.80  
  Purchase #5 $76.87   10.263%   $84.7592   $84.76   $84.75   $84.80  
  Purchase #6 $9.79   13.737%   $11.1349   $11.13   $11.15   $11.10  
  Purchase #7 $155.37   4.497%   $162.357   $162.36   $162.35   $162.40  
  7 purchase totals = $485.95       $518.5298   $518.53   $518.50   $518.60