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	$17.75	6.335%	$18.8745	$18.87	$18.85	$18.90
Purchase #2	$44.57	9.473%	$48.7921	$48.79	$48.80	$48.80
Purchase #3	$132.62	11.658%	$148.0808	$148.08	$148.10	$148.10
Purchase #4	$130.89	13.145%	$148.0955	$148.10	$148.10	$148.10
Purchase #5	$66.69	8.485%	$72.3486	$72.35	$72.35	$72.30
Purchase #6	$241.30	3.529%	$249.8155	$249.82	$249.80	$249.80
Purchase #7	$239.89	6.233%	$254.8423	$254.84	$254.85	$254.80
7 purchase totals =	$873.71		$940.8493	$940.85	$940.85	$940.80