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	$16.60	11.472%	$18.5044	$18.50	$18.50	$18.50
Purchase #2	$151.90	13.638%	$172.6161	$172.62	$172.60	$172.60
Purchase #3	$215.78	13.557%	$245.0333	$245.03	$245.05	$245.00
Purchase #4	$191.79	5.786%	$202.887	$202.89	$202.90	$202.90
Purchase #5	$9.42	11.824%	$10.5338	$10.53	$10.55	$10.50
Purchase #6	$168.27	11.592%	$187.7759	$187.78	$187.80	$187.80
Purchase #7	$131.44	7.246%	$140.9641	$140.96	$140.95	$141.00
7 purchase totals =	$885.20		$978.3146	$978.31	$978.35	$978.30