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	$80.58	11.194%	$89.6001	$89.60	$89.60	$89.60
Purchase #2	$104.56	4.566%	$109.3342	$109.33	$109.35	$109.30
Purchase #3	$51.50	12.874%	$58.1301	$58.13	$58.15	$58.10
Purchase #4	$186.87	10.859%	$207.1622	$207.16	$207.15	$207.20
Purchase #5	$62.46	6.733%	$66.6654	$66.67	$66.65	$66.70
Purchase #6	$6.81	9.405%	$7.4505	$7.45	$7.45	$7.50
Purchase #7	$107.78	3.651%	$111.715	$111.72	$111.70	$111.70
7 purchase totals =	$600.56		$650.0575	$650.06	$650.05	$650.10