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	$18.87	11.515%	$21.0429	$21.04	$21.05	$21.00
Purchase #2	$25.50	11.789%	$28.5062	$28.51	$28.50	$28.50
Purchase #3	$105.70	3.216%	$109.0993	$109.10	$109.10	$109.10
Purchase #4	$99.82	4.797%	$104.6084	$104.61	$104.60	$104.60
Purchase #5	$13.39	3.683%	$13.8832	$13.88	$13.90	$13.90
Purchase #6	$117.81	4.531%	$123.148	$123.15	$123.15	$123.10
Purchase #7	$109.45	11.174%	$121.6799	$121.68	$121.70	$121.70
7 purchase totals =	$490.54		$521.9679	$521.97	$522.00	$521.90