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	$5.80	6.174%	$6.1581	$6.16	$6.15	$6.20
Purchase #2	$48.25	11.554%	$53.8248	$53.82	$53.80	$53.80
Purchase #3	$124.76	9.565%	$136.6933	$136.69	$136.70	$136.70
Purchase #4	$27.83	6.158%	$29.5438	$29.54	$29.55	$29.50
Purchase #5	$147.82	3.872%	$153.5436	$153.54	$153.55	$153.50
Purchase #6	$54.72	9.137%	$59.7198	$59.72	$59.70	$59.70
Purchase #7	$178.43	11.869%	$199.6079	$199.61	$199.60	$199.60
7 purchase totals =	$587.61		$639.0913	$639.08	$639.05	$639.00