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	$16.68	8.778%	$18.1442	$18.14	$18.15	$18.10
Purchase #2	$126.61	13.196%	$143.3175	$143.32	$143.30	$143.30
Purchase #3	$72.56	9.189%	$79.2275	$79.23	$79.25	$79.20
Purchase #4	$29.51	4.619%	$30.8731	$30.87	$30.85	$30.90
Purchase #5	$72.93	10.580%	$80.646	$80.65	$80.65	$80.60
Purchase #6	$215.73	6.255%	$229.2239	$229.22	$229.20	$229.20
Purchase #7	$70.70	11.305%	$78.6926	$78.69	$78.70	$78.70
7 purchase totals =	$604.72		$660.1248	$660.12	$660.10	$660.00