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 $12.53   3.709%   $12.9947   $12.99   $13.00   $13.00  
  Purchase #2 $26.84   12.302%   $30.1419   $30.14   $30.15   $30.10  
  Purchase #3 $19.95   5.545%   $21.0562   $21.06   $21.05   $21.10  
  Purchase #4 $100.39   11.815%   $112.2511   $112.25   $112.25   $112.30  
  Purchase #5 $228.76   13.714%   $260.1321   $260.13   $260.15   $260.10  
  Purchase #6 $77.45   5.838%   $81.9715   $81.97   $81.95   $82.00  
  Purchase #7 $90.62   6.726%   $96.7151   $96.72   $96.70   $96.70  
  7 purchase totals = $556.54       $615.2626   $615.26   $615.25   $615.30