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	$5.52	8.311%	$5.9788	$5.98	$6.00	$6.00
Purchase #2	$5.76	6.661%	$6.1437	$6.14	$6.15	$6.10
Purchase #3	$118.61	6.623%	$126.4655	$126.47	$126.45	$126.50
Purchase #4	$94.92	9.386%	$103.8292	$103.83	$103.85	$103.80
Purchase #5	$108.26	3.290%	$111.8218	$111.82	$111.80	$111.80
Purchase #6	$28.57	3.647%	$29.6119	$29.61	$29.60	$29.60
Purchase #7	$169.56	10.353%	$187.1145	$187.11	$187.10	$187.10
7 purchase totals =	$531.20		$570.9654	$570.96	$570.95	$570.90