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 $6.30   9.269%   $6.8839   $6.88   $6.90   $6.90  
  Purchase #2 $14.69   9.242%   $16.0476   $16.05   $16.05   $16.00  
  Purchase #3 $242.31   12.597%   $272.8338   $272.83   $272.85   $272.80  
  Purchase #4 $121.42   9.526%   $132.9865   $132.99   $133.00   $133.00  
  Purchase #5 $127.75   12.296%   $143.4581   $143.46   $143.45   $143.50  
  Purchase #6 $83.51   3.867%   $86.7393   $86.74   $86.75   $86.70  
  Purchase #7 $50.53   13.800%   $57.5031   $57.50   $57.50   $57.50  
  7 purchase totals = $646.51       $716.4523   $716.45   $716.50   $716.40