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 $25.73   10.718%   $28.4877   $28.49   $28.50   $28.50  
  Purchase #2 $46.30   5.837%   $49.0025   $49.00   $49.00   $49.00  
  Purchase #3 $237.61   6.242%   $252.4416   $252.44   $252.45   $252.40  
  Purchase #4 $125.97   12.295%   $141.458   $141.46   $141.45   $141.50  
  Purchase #5 $23.57   4.465%   $24.6224   $24.62   $24.60   $24.60  
  Purchase #6 $59.43   3.837%   $61.7103   $61.71   $61.70   $61.70  
  Purchase #7 $21.76   11.346%   $24.2289   $24.23   $24.25   $24.20  
  7 purchase totals = $540.37       $581.9514   $581.95   $581.95   $581.90