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 $239.25   6.836%   $255.6051   $255.61   $255.60   $255.60  
  Purchase #2 $176.42   4.298%   $184.0025   $184.00   $184.00   $184.00  
  Purchase #3 $135.50   10.243%   $149.3793   $149.38   $149.40   $149.40  
  Purchase #4 $154.25   9.246%   $168.512   $168.51   $168.50   $168.50  
  Purchase #5 $244.36   5.231%   $257.1425   $257.14   $257.15   $257.10  
  Purchase #6 $107.60   3.602%   $111.4758   $111.48   $111.50   $111.50  
  Purchase #7 $135.42   13.373%   $153.5297   $153.53   $153.55   $153.50  
  7 purchase totals = $1,192.80       $1,279.6469   $1,279.65   $1,279.70   $1,279.60