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 $239.96   12.417%   $269.7558   $269.76   $269.75   $269.80  
  Purchase #2 $49.69   3.646%   $51.5017   $51.50   $51.50   $51.50  
  Purchase #3 $216.56   9.287%   $236.6719   $236.67   $236.65   $236.70  
  Purchase #4 $241.60   12.152%   $270.9592   $270.96   $270.95   $271.00  
  Purchase #5 $246.43   6.260%   $261.8565   $261.86   $261.85   $261.90  
  Purchase #6 $88.61   12.731%   $99.8909   $99.89   $99.90   $99.90  
  Purchase #7 $130.49   6.297%   $138.707   $138.71   $138.70   $138.70  
  7 purchase totals = $1,213.34       $1,329.3430   $1,329.35   $1,329.30   $1,329.50