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 $235.99   10.585%   $260.9695   $260.97   $260.95   $261.00  
  Purchase #2 $248.85   8.837%   $270.8409   $270.84   $270.85   $270.80  
  Purchase #3 $41.94   11.418%   $46.7287   $46.73   $46.75   $46.70  
  Purchase #4 $125.88   5.284%   $132.5315   $132.53   $132.55   $132.50  
  Purchase #5 $11.39   6.662%   $12.1488   $12.15   $12.15   $12.10  
  Purchase #6 $81.69   8.263%   $88.44   $88.44   $88.45   $88.40  
  Purchase #7 $27.31   12.780%   $30.8002   $30.80   $30.80   $30.80  
  7 purchase totals = $773.05       $842.4596   $842.46   $842.50   $842.30