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 $20.67   4.777%   $21.6574   $21.66   $21.65   $21.70  
  Purchase #2 $125.98   4.468%   $131.6088   $131.61   $131.60   $131.60  
  Purchase #3 $101.74   3.466%   $105.2663   $105.27   $105.25   $105.30  
  Purchase #4 $169.42   7.679%   $182.4298   $182.43   $182.45   $182.40  
  Purchase #5 $163.76   8.643%   $177.9138   $177.91   $177.90   $177.90  
  Purchase #6 $178.39   8.284%   $193.1678   $193.17   $193.15   $193.20  
  Purchase #7 $104.35   3.200%   $107.6892   $107.69   $107.70   $107.70  
  7 purchase totals = $864.31       $919.7331   $919.74   $919.70   $919.80