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 $249.41   8.134%   $269.697   $269.70   $269.70   $269.70  
  Purchase #2 $18.39   13.511%   $20.8747   $20.87   $20.85   $20.90  
  Purchase #3 $163.36   12.427%   $183.6607   $183.66   $183.65   $183.70  
  Purchase #4 $236.56   8.535%   $256.7504   $256.75   $256.75   $256.80  
  Purchase #5 $195.70   12.261%   $219.6948   $219.69   $219.70   $219.70  
  Purchase #6 $238.88   9.472%   $261.5067   $261.51   $261.50   $261.50  
  Purchase #7 $214.91   10.362%   $237.179   $237.18   $237.20   $237.20  
  7 purchase totals = $1,317.21       $1,449.3633   $1,449.36   $1,449.35   $1,449.50