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	$34.94	11.787%	$39.0584	$39.06	$39.05	$39.10
Purchase #2	$183.49	5.652%	$193.8609	$193.86	$193.85	$193.90
Purchase #3	$207.96	12.634%	$234.2337	$234.23	$234.25	$234.20
Purchase #4	$102.41	3.784%	$106.2852	$106.29	$106.30	$106.30
Purchase #5	$56.78	4.721%	$59.4606	$59.46	$59.45	$59.50
Purchase #6	$102.68	6.431%	$109.2834	$109.28	$109.30	$109.30
Purchase #7	$122.70	4.171%	$127.8178	$127.82	$127.80	$127.80
7 purchase totals =	$810.96		$870.0000	$870.00	$870.00	$870.10