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	$37.83	3.385%	$39.1105	$39.11	$39.10	$39.10
Purchase #2	$82.92	8.162%	$89.6879	$89.69	$89.70	$89.70
Purchase #3	$142.44	11.106%	$158.2594	$158.26	$158.25	$158.30
Purchase #4	$90.65	6.474%	$96.5187	$96.52	$96.50	$96.50
Purchase #5	$96.77	10.593%	$107.0208	$107.02	$107.00	$107.00
Purchase #6	$47.62	4.462%	$49.7448	$49.74	$49.75	$49.70
Purchase #7	$91.72	13.704%	$104.2893	$104.29	$104.30	$104.30
7 purchase totals =	$589.95		$644.6314	$644.63	$644.60	$644.60