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	$18.93	9.726%	$20.7711	$20.77	$20.75	$20.80
Purchase #2	$139.72	3.332%	$144.3755	$144.38	$144.40	$144.40
Purchase #3	$34.72	10.705%	$38.4368	$38.44	$38.45	$38.40
Purchase #4	$12.47	9.454%	$13.6489	$13.65	$13.65	$13.60
Purchase #5	$122.64	4.487%	$128.1429	$128.14	$128.15	$128.10
Purchase #6	$11.90	6.226%	$12.6409	$12.64	$12.65	$12.60
Purchase #7	$205.85	10.478%	$227.419	$227.42	$227.40	$227.40
7 purchase totals =	$546.23		$585.4351	$585.44	$585.45	$585.30