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 $20.43   3.552%   $21.1557   $21.16   $21.15   $21.20  
  Purchase #2 $42.71   8.697%   $46.4245   $46.42   $46.40   $46.40  
  Purchase #3 $216.79   10.712%   $240.0125   $240.01   $240.00   $240.00  
  Purchase #4 $122.46   5.806%   $129.57   $129.57   $129.55   $129.60  
  Purchase #5 $22.55   9.702%   $24.7378   $24.74   $24.75   $24.70  
  Purchase #6 $50.58   11.805%   $56.551   $56.55   $56.55   $56.60  
  Purchase #7 $20.64   13.410%   $23.4078   $23.41   $23.40   $23.40  
  7 purchase totals = $496.16       $541.8593   $541.86   $541.80   $541.90