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	$18.98	11.222%	$21.1099	$21.11	$21.10	$21.10
Purchase #2	$100.82	5.750%	$106.6172	$106.62	$106.60	$106.60
Purchase #3	$101.71	3.308%	$105.0746	$105.07	$105.05	$105.10
Purchase #4	$109.50	10.467%	$120.9614	$120.96	$120.95	$121.00
Purchase #5	$107.88	9.521%	$118.1513	$118.15	$118.15	$118.20
Purchase #6	$58.29	13.290%	$66.0367	$66.04	$66.05	$66.00
Purchase #7	$116.86	10.694%	$129.357	$129.36	$129.35	$129.40
7 purchase totals =	$614.04		$667.3081	$667.31	$667.25	$667.40