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 $17.55   13.838%   $19.9786   $19.98   $20.00   $20.00  
  Purchase #2 $112.66   3.176%   $116.2381   $116.24   $116.25   $116.20  
  Purchase #3 $10.90   9.567%   $11.9428   $11.94   $11.95   $11.90  
  Purchase #4 $245.55   4.828%   $257.4052   $257.41   $257.40   $257.40  
  Purchase #5 $149.59   9.733%   $164.1496   $164.15   $164.15   $164.10  
  Purchase #6 $62.61   13.650%   $71.1563   $71.16   $71.15   $71.20  
  Purchase #7 $43.87   13.821%   $49.9333   $49.93   $49.95   $49.90  
  7 purchase totals = $642.73       $690.8039   $690.81   $690.85   $690.70