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 $60.97   12.677%   $68.6992   $68.70   $68.70   $68.70  
  Purchase #2 $8.76   9.324%   $9.5768   $9.58   $9.60   $9.60  
  Purchase #3 $53.93   12.749%   $60.8055   $60.81   $60.80   $60.80  
  Purchase #4 $115.42   8.558%   $125.2976   $125.30   $125.30   $125.30  
  Purchase #5 $45.73   12.504%   $51.4481   $51.45   $51.45   $51.40  
  Purchase #6 $27.75   10.180%   $30.575   $30.58   $30.60   $30.60  
  Purchase #7 $163.98   11.821%   $183.3641   $183.36   $183.35   $183.40  
  7 purchase totals = $476.54       $529.7663   $529.78   $529.80   $529.80