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 $250.27   11.855%   $279.9395   $279.94   $279.95   $279.90  
  Purchase #2 $236.94   13.458%   $268.8274   $268.83   $268.85   $268.80  
  Purchase #3 $61.41   6.250%   $65.2481   $65.25   $65.25   $65.20  
  Purchase #4 $124.71   13.763%   $141.8738   $141.87   $141.85   $141.90  
  Purchase #5 $95.89   7.344%   $102.9322   $102.93   $102.95   $102.90  
  Purchase #6 $199.72   3.549%   $206.8081   $206.81   $206.80   $206.80  
  Purchase #7 $135.62   3.459%   $140.3111   $140.31   $140.30   $140.30  
  7 purchase totals = $1,104.56       $1,205.9402   $1,205.94   $1,205.95   $1,205.80