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 $49.51   6.121%   $52.5405   $52.54   $52.55   $52.50  
  Purchase #2 $247.41   13.559%   $280.9563   $280.96   $280.95   $281.00  
  Purchase #3 $34.49   3.224%   $35.602   $35.60   $35.60   $35.60  
  Purchase #4 $75.97   8.858%   $82.6994   $82.70   $82.70   $82.70  
  Purchase #5 $50.38   9.632%   $55.2326   $55.23   $55.25   $55.20  
  Purchase #6 $248.60   13.296%   $281.6539   $281.65   $281.65   $281.70  
  Purchase #7 $21.46   11.862%   $24.0056   $24.01   $24.00   $24.00  
  7 purchase totals = $727.82       $812.6903   $812.69   $812.70   $812.70