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	$10.60	4.785%	$11.1072	$11.11	$11.10	$11.10
Purchase #2	$103.77	7.438%	$111.4884	$111.49	$111.50	$111.50
Purchase #3	$246.28	5.117%	$258.8821	$258.88	$258.90	$258.90
Purchase #4	$158.83	12.740%	$179.0649	$179.06	$179.05	$179.10
Purchase #5	$205.39	13.857%	$233.8509	$233.85	$233.85	$233.90
Purchase #6	$17.39	12.814%	$19.6184	$19.62	$19.60	$19.60
Purchase #7	$23.80	8.725%	$25.8766	$25.88	$25.90	$25.90
7 purchase totals =	$766.06		$839.8885	$839.89	$839.90	$840.00