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 $99.99   5.872%   $105.8614   $105.86   $105.85   $105.90  
  Purchase #2 $84.88   10.517%   $93.8068   $93.81   $93.80   $93.80  
  Purchase #3 $214.97   3.629%   $222.7713   $222.77   $222.75   $222.80  
  Purchase #4 $113.88   9.814%   $125.0562   $125.06   $125.05   $125.10  
  Purchase #5 $190.84   11.693%   $213.1549   $213.15   $213.15   $213.20  
  Purchase #6 $51.76   6.500%   $55.1244   $55.12   $55.10   $55.10  
  Purchase #7 $146.25   12.233%   $164.1408   $164.14   $164.15   $164.10  
  7 purchase totals = $902.57       $979.9158   $979.91   $979.85   $980.00