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	$60.63	9.314%	$66.2771	$66.28	$66.30	$66.30
Purchase #2	$231.71	8.583%	$251.5977	$251.60	$251.60	$251.60
Purchase #3	$98.45	5.388%	$103.7545	$103.75	$103.75	$103.80
Purchase #4	$143.54	11.827%	$160.5165	$160.52	$160.50	$160.50
Purchase #5	$85.49	6.255%	$90.8374	$90.84	$90.85	$90.80
Purchase #6	$85.89	10.282%	$94.7212	$94.72	$94.70	$94.70
Purchase #7	$64.75	11.424%	$72.147	$72.15	$72.15	$72.10
7 purchase totals =	$770.46		$839.8514	$839.86	$839.85	$839.80