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	$36.86	7.380%	$39.5803	$39.58	$39.60	$39.60
Purchase #2	$33.69	4.235%	$35.1168	$35.12	$35.10	$35.10
Purchase #3	$54.62	7.565%	$58.752	$58.75	$58.75	$58.80
Purchase #4	$52.89	11.449%	$58.9454	$58.95	$58.95	$58.90
Purchase #5	$122.86	4.127%	$127.9304	$127.93	$127.95	$127.90
Purchase #6	$232.85	3.418%	$240.8088	$240.81	$240.80	$240.80
Purchase #7	$74.29	5.603%	$78.4525	$78.45	$78.45	$78.50
7 purchase totals =	$608.06		$639.5862	$639.59	$639.60	$639.60