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 $14.41   12.504%   $16.2118   $16.21   $16.20   $16.20  
  Purchase #2 $219.69   7.725%   $236.6611   $236.66   $236.65   $236.70  
  Purchase #3 $59.49   3.212%   $61.4008   $61.40   $61.40   $61.40  
  Purchase #4 $18.45   10.444%   $20.3769   $20.38   $20.40   $20.40  
  Purchase #5 $27.79   6.504%   $29.5975   $29.60   $29.60   $29.60  
  Purchase #6 $11.44   7.370%   $12.2831   $12.28   $12.30   $12.30  
  Purchase #7 $190.89   7.607%   $205.411   $205.41   $205.40   $205.40  
  7 purchase totals = $542.16       $581.9422   $581.94   $581.95   $582.00