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	$18.90	7.354%	$20.2899	$20.29	$20.30	$20.30
Purchase #2	$240.25	12.577%	$270.4662	$270.47	$270.45	$270.50
Purchase #3	$119.72	6.283%	$127.242	$127.24	$127.25	$127.20
Purchase #4	$238.97	13.841%	$272.0458	$272.05	$272.05	$272.00
Purchase #5	$135.86	4.446%	$141.9003	$141.90	$141.90	$141.90
Purchase #6	$196.89	10.245%	$217.0614	$217.06	$217.05	$217.10
Purchase #7	$247.99	12.755%	$279.6211	$279.62	$279.60	$279.60
7 purchase totals =	$1,198.58		$1,328.6267	$1,328.63	$1,328.60	$1,328.60