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	$29.50	5.423%	$31.0998	$31.10	$31.10	$31.10
Purchase #2	$114.92	7.557%	$123.6045	$123.60	$123.60	$123.60
Purchase #3	$38.83	8.267%	$42.0401	$42.04	$42.05	$42.00
Purchase #4	$79.54	12.831%	$89.7458	$89.75	$89.75	$89.70
Purchase #5	$140.70	6.330%	$149.6063	$149.61	$149.60	$149.60
Purchase #6	$14.83	8.557%	$16.099	$16.10	$16.10	$16.10
Purchase #7	$176.67	6.131%	$187.5016	$187.50	$187.50	$187.50
7 purchase totals =	$594.99		$639.6971	$639.70	$639.70	$639.60