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 $125.67   11.123%   $139.6483   $139.65   $139.65   $139.60  
  Purchase #2 $249.34   4.262%   $259.9669   $259.97   $259.95   $260.00  
  Purchase #3 $26.81   11.865%   $29.991   $29.99   $30.00   $30.00  
  Purchase #4 $248.46   5.343%   $261.7352   $261.74   $261.75   $261.70  
  Purchase #5 $142.94   5.147%   $150.2971   $150.30   $150.30   $150.30  
  Purchase #6 $75.44   8.422%   $81.7936   $81.79   $81.80   $81.80  
  Purchase #7 $82.46   9.528%   $90.3168   $90.32   $90.30   $90.30  
  7 purchase totals = $951.12       $1,013.7489   $1,013.76   $1,013.75   $1,013.70