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	$6.68	7.518%	$7.1822	$7.18	$7.20	$7.20
Purchase #2	$38.77	8.812%	$42.1864	$42.19	$42.20	$42.20
Purchase #3	$136.60	6.730%	$145.7932	$145.79	$145.80	$145.80
Purchase #4	$108.92	13.401%	$123.5164	$123.52	$123.50	$123.50
Purchase #5	$239.30	6.397%	$254.608	$254.61	$254.60	$254.60
Purchase #6	$35.66	7.741%	$38.4204	$38.42	$38.40	$38.40
Purchase #7	$26.58	3.746%	$27.5757	$27.58	$27.60	$27.60
7 purchase totals =	$592.51		$639.2823	$639.29	$639.30	$639.30