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 $248.39   8.390%   $269.2299   $269.23   $269.25   $269.20  
  Purchase #2 $227.74   4.817%   $238.7102   $238.71   $238.70   $238.70  
  Purchase #3 $174.64   5.789%   $184.7499   $184.75   $184.75   $184.70  
  Purchase #4 $42.91   8.785%   $46.6796   $46.68   $46.70   $46.70  
  Purchase #5 $127.27   10.640%   $140.8115   $140.81   $140.80   $140.80  
  Purchase #6 $121.89   12.214%   $136.7776   $136.78   $136.80   $136.80  
  Purchase #7 $21.47   6.578%   $22.8823   $22.88   $22.90   $22.90  
  7 purchase totals = $964.31       $1,039.8410   $1,039.84   $1,039.90   $1,039.80