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	$68.74	13.802%	$78.2275	$78.23	$78.25	$78.20
Purchase #2	$55.78	7.160%	$59.7738	$59.77	$59.75	$59.80
Purchase #3	$160.76	7.339%	$172.5582	$172.56	$172.55	$172.60
Purchase #4	$12.92	8.858%	$14.0645	$14.06	$14.05	$14.10
Purchase #5	$15.39	12.289%	$17.2813	$17.28	$17.30	$17.30
Purchase #6	$225.50	8.169%	$243.9211	$243.92	$243.90	$243.90
Purchase #7	$47.67	11.640%	$53.2188	$53.22	$53.20	$53.20
7 purchase totals =	$586.76		$639.0452	$639.04	$639.00	$639.10