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	$26.68	12.863%	$30.1118	$30.11	$30.10	$30.10
Purchase #2	$123.31	12.298%	$138.4747	$138.47	$138.45	$138.50
Purchase #3	$40.46	4.392%	$42.237	$42.24	$42.25	$42.20
Purchase #4	$101.51	10.196%	$111.86	$111.86	$111.85	$111.90
Purchase #5	$49.29	13.267%	$55.8293	$55.83	$55.85	$55.80
Purchase #6	$66.97	8.304%	$72.5312	$72.53	$72.55	$72.50
Purchase #7	$5.26	4.189%	$5.4803	$5.48	$5.50	$5.50
7 purchase totals =	$413.48		$456.5243	$456.52	$456.55	$456.50