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	$24.83	13.177%	$28.1018	$28.10	$28.10	$28.10
Purchase #2	$120.78	8.193%	$130.6755	$130.68	$130.70	$130.70
Purchase #3	$24.85	11.317%	$27.6623	$27.66	$27.65	$27.70
Purchase #4	$111.81	13.796%	$127.2353	$127.24	$127.25	$127.20
Purchase #5	$48.46	10.739%	$53.6641	$53.66	$53.65	$53.70
Purchase #6	$52.42	4.340%	$54.695	$54.70	$54.70	$54.70
Purchase #7	$215.82	4.640%	$225.834	$225.83	$225.85	$225.80
7 purchase totals =	$598.97		$647.8680	$647.87	$647.90	$647.90