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 $75.50   4.617%   $78.9858   $78.99   $79.00   $79.00  
  Purchase #2 $104.90   4.149%   $109.2523   $109.25   $109.25   $109.30  
  Purchase #3 $127.81   12.388%   $143.6431   $143.64   $143.65   $143.60  
  Purchase #4 $47.42   11.773%   $53.0028   $53.00   $53.00   $53.00  
  Purchase #5 $171.39   7.639%   $184.4825   $184.48   $184.50   $184.50  
  Purchase #6 $41.63   3.234%   $42.9763   $42.98   $43.00   $43.00  
  Purchase #7 $62.80   5.102%   $66.0041   $66.00   $66.00   $66.00  
  7 purchase totals = $631.45       $678.3469   $678.34   $678.40   $678.40