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	$20.66	5.673%	$21.832	$21.83	$21.85	$21.80
Purchase #2	$199.86	4.783%	$209.4193	$209.42	$209.40	$209.40
Purchase #3	$73.97	11.855%	$82.7391	$82.74	$82.75	$82.70
Purchase #4	$39.52	6.653%	$42.1493	$42.15	$42.15	$42.10
Purchase #5	$166.64	10.264%	$183.7439	$183.74	$183.75	$183.70
Purchase #6	$154.45	8.249%	$167.1906	$167.19	$167.20	$167.20
Purchase #7	$127.82	4.510%	$133.5847	$133.58	$133.60	$133.60
7 purchase totals =	$782.92		$840.6589	$840.65	$840.70	$840.50