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	$109.60	12.783%	$123.6102	$123.61	$123.60	$123.60
Purchase #2	$124.78	12.613%	$140.5185	$140.52	$140.50	$140.50
Purchase #3	$128.66	11.551%	$143.5215	$143.52	$143.50	$143.50
Purchase #4	$94.36	4.585%	$98.6864	$98.69	$98.70	$98.70
Purchase #5	$20.29	12.782%	$22.8835	$22.88	$22.90	$22.90
Purchase #6	$52.82	9.172%	$57.6647	$57.66	$57.65	$57.70
Purchase #7	$47.40	11.571%	$52.8847	$52.88	$52.90	$52.90
7 purchase totals =	$577.91		$639.7695	$639.76	$639.75	$639.80