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 $16.72   8.615%   $18.1604   $18.16   $18.15   $18.20  
  Purchase #2 $40.84   9.724%   $44.8113   $44.81   $44.80   $44.80  
  Purchase #3 $223.68   6.823%   $238.9417   $238.94   $238.95   $238.90  
  Purchase #4 $5.50   8.659%   $5.9762   $5.98   $6.00   $6.00  
  Purchase #5 $237.25   5.326%   $249.8859   $249.89   $249.90   $249.90  
  Purchase #6 $5.96   11.369%   $6.6376   $6.64   $6.65   $6.60  
  Purchase #7 $63.93   6.738%   $68.2376   $68.24   $68.25   $68.20  
  7 purchase totals = $593.88       $632.6507   $632.66   $632.70   $632.60