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	$19.60	8.806%	$21.326	$21.33	$21.35	$21.30
Purchase #2	$57.91	4.510%	$60.5217	$60.52	$60.50	$60.50
Purchase #3	$240.65	7.237%	$258.0658	$258.07	$258.05	$258.10
Purchase #4	$88.26	6.404%	$93.9122	$93.91	$93.90	$93.90
Purchase #5	$99.42	12.281%	$111.6298	$111.63	$111.65	$111.60
Purchase #6	$77.56	6.677%	$82.7387	$82.74	$82.75	$82.70
Purchase #7	$14.32	11.347%	$15.9449	$15.94	$15.95	$15.90
7 purchase totals =	$597.72		$644.1391	$644.14	$644.15	$644.00