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	$15.84	13.585%	$17.9919	$17.99	$18.00	$18.00
Purchase #2	$131.79	8.615%	$143.1437	$143.14	$143.15	$143.10
Purchase #3	$85.53	4.200%	$89.1223	$89.12	$89.10	$89.10
Purchase #4	$29.42	8.795%	$32.0075	$32.01	$32.00	$32.00
Purchase #5	$96.61	12.129%	$108.3278	$108.33	$108.35	$108.30
Purchase #6	$243.73	4.739%	$255.2804	$255.28	$255.30	$255.30
Purchase #7	$74.37	5.480%	$78.4455	$78.45	$78.45	$78.40
7 purchase totals =	$677.29		$724.3191	$724.32	$724.35	$724.20