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	$18.67	12.699%	$21.0409	$21.04	$21.05	$21.00
Purchase #2	$136.83	8.439%	$148.3771	$148.38	$148.40	$148.40
Purchase #3	$183.47	7.297%	$196.8578	$196.86	$196.85	$196.90
Purchase #4	$12.55	6.511%	$13.3671	$13.37	$13.35	$13.40
Purchase #5	$199.83	6.638%	$213.0947	$213.09	$213.10	$213.10
Purchase #6	$134.76	6.551%	$143.5881	$143.59	$143.60	$143.60
Purchase #7	$178.38	4.859%	$187.0475	$187.05	$187.05	$187.00
7 purchase totals =	$864.49		$923.3732	$923.38	$923.40	$923.40