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 $238.92   8.402%   $258.9941   $258.99   $259.00   $259.00  
  Purchase #2 $144.94   6.808%   $154.8075   $154.81   $154.80   $154.80  
  Purchase #3 $64.27   5.301%   $67.677   $67.68   $67.70   $67.70  
  Purchase #4 $68.37   11.491%   $76.2264   $76.23   $76.25   $76.20  
  Purchase #5 $249.60   12.419%   $280.5978   $280.60   $280.60   $280.60  
  Purchase #6 $100.27   4.319%   $104.6007   $104.60   $104.60   $104.60  
  Purchase #7 $24.70   6.361%   $26.2712   $26.27   $26.25   $26.30  
  7 purchase totals = $891.07       $969.1747   $969.18   $969.20   $969.20