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	$35.98	8.655%	$39.0941	$39.09	$39.10	$39.10
Purchase #2	$210.75	9.521%	$230.8155	$230.82	$230.80	$230.80
Purchase #3	$36.69	10.307%	$40.4716	$40.47	$40.45	$40.50
Purchase #4	$22.96	11.496%	$25.5995	$25.60	$25.60	$25.60
Purchase #5	$81.83	10.538%	$90.4532	$90.45	$90.45	$90.50
Purchase #6	$43.28	13.683%	$49.202	$49.20	$49.20	$49.20
Purchase #7	$148.78	9.147%	$162.3889	$162.39	$162.40	$162.40
7 purchase totals =	$580.27		$638.0248	$638.02	$638.00	$638.10