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	$240.83	6.255%	$255.8939	$255.89	$255.90	$255.90
Purchase #2	$140.92	3.782%	$146.2496	$146.25	$146.25	$146.20
Purchase #3	$48.53	4.239%	$50.5872	$50.59	$50.60	$50.60
Purchase #4	$180.64	7.814%	$194.7552	$194.76	$194.75	$194.80
Purchase #5	$229.29	11.268%	$255.1264	$255.13	$255.15	$255.10
Purchase #6	$226.78	8.634%	$246.3602	$246.36	$246.35	$246.40
Purchase #7	$144.98	7.197%	$155.4142	$155.41	$155.40	$155.40
7 purchase totals =	$1,211.97		$1,304.3867	$1,304.39	$1,304.40	$1,304.40