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	$7.88	6.243%	$8.3719	$8.37	$8.35	$8.40
Purchase #2	$143.62	6.403%	$152.816	$152.82	$152.80	$152.80
Purchase #3	$36.27	12.722%	$40.8843	$40.88	$40.90	$40.90
Purchase #4	$71.65	10.837%	$79.4147	$79.41	$79.40	$79.40
Purchase #5	$107.47	8.204%	$116.2868	$116.29	$116.30	$116.30
Purchase #6	$198.70	11.521%	$221.5922	$221.59	$221.60	$221.60
Purchase #7	$55.75	7.346%	$59.8454	$59.85	$59.85	$59.80
7 purchase totals =	$621.34		$679.2113	$679.21	$679.20	$679.20