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 $21.72   4.516%   $22.7009   $22.70   $22.70   $22.70  
  Purchase #2 $190.47   5.325%   $200.6125   $200.61   $200.60   $200.60  
  Purchase #3 $15.92   13.613%   $18.0872   $18.09   $18.10   $18.10  
  Purchase #4 $144.75   4.129%   $150.7267   $150.73   $150.75   $150.70  
  Purchase #5 $15.89   8.195%   $17.1922   $17.19   $17.20   $17.20  
  Purchase #6 $18.30   7.466%   $19.6663   $19.67   $19.65   $19.70  
  Purchase #7 $216.83   8.538%   $235.3429   $235.34   $235.35   $235.30  
  7 purchase totals = $623.88       $664.3287   $664.33   $664.35   $664.30