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 $244.92   10.162%   $269.8088   $269.81   $269.80   $269.80  
  Purchase #2 $115.76   8.767%   $125.9087   $125.91   $125.90   $125.90  
  Purchase #3 $30.55   8.260%   $33.0734   $33.07   $33.05   $33.10  
  Purchase #4 $223.77   4.833%   $234.5848   $234.58   $234.60   $234.60  
  Purchase #5 $17.29   11.233%   $19.2322   $19.23   $19.25   $19.20  
  Purchase #6 $150.63   9.258%   $164.5753   $164.58   $164.60   $164.60  
  Purchase #7 $132.80   10.239%   $146.3974   $146.40   $146.40   $146.40  
  7 purchase totals = $915.72       $993.5806   $993.58   $993.60   $993.60