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	$18.70	9.746%	$20.5225	$20.52	$20.50	$20.50
Purchase #2	$33.92	4.377%	$35.4047	$35.40	$35.40	$35.40
Purchase #3	$76.77	4.763%	$80.4266	$80.43	$80.45	$80.40
Purchase #4	$114.73	13.800%	$130.5627	$130.56	$130.55	$130.60
Purchase #5	$158.55	4.702%	$166.005	$166.01	$166.00	$166.00
Purchase #6	$171.38	13.872%	$195.1538	$195.15	$195.15	$195.20
Purchase #7	$11.37	4.469%	$11.8781	$11.88	$11.90	$11.90
7 purchase totals =	$585.42		$639.9534	$639.95	$639.95	$640.00