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 $191.89   13.247%   $217.3097   $217.31   $217.30   $217.30  
  Purchase #2 $95.57   9.689%   $104.8298   $104.83   $104.85   $104.80  
  Purchase #3 $60.26   5.696%   $63.6924   $63.69   $63.70   $63.70  
  Purchase #4 $17.69   12.628%   $19.9239   $19.92   $19.90   $19.90  
  Purchase #5 $237.25   3.691%   $246.0069   $246.01   $246.00   $246.00  
  Purchase #6 $83.90   9.520%   $91.8873   $91.89   $91.90   $91.90  
  Purchase #7 $213.39   10.443%   $235.6743   $235.67   $235.65   $235.70  
  7 purchase totals = $899.95       $979.3243   $979.32   $979.30   $979.30