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	$13.66	5.775%	$14.4489	$14.45	$14.45	$14.40
Purchase #2	$129.88	8.439%	$140.8406	$140.84	$140.85	$140.80
Purchase #3	$56.37	8.679%	$61.2624	$61.26	$61.25	$61.30
Purchase #4	$93.76	10.487%	$103.5926	$103.59	$103.60	$103.60
Purchase #5	$114.92	6.433%	$122.3128	$122.31	$122.30	$122.30
Purchase #6	$132.86	6.789%	$141.8799	$141.88	$141.90	$141.90
Purchase #7	$52.33	5.635%	$55.2788	$55.28	$55.30	$55.30
7 purchase totals =	$593.78		$639.6160	$639.61	$639.65	$639.60