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	$91.40	9.389%	$99.9815	$99.98	$100.00	$100.00
Purchase #2	$112.83	9.700%	$123.7745	$123.77	$123.75	$123.80
Purchase #3	$21.64	13.369%	$24.5331	$24.53	$24.55	$24.50
Purchase #4	$11.41	11.441%	$12.7154	$12.72	$12.70	$12.70
Purchase #5	$246.37	13.657%	$280.0168	$280.02	$280.00	$280.00
Purchase #6	$23.81	8.566%	$25.8496	$25.85	$25.85	$25.80
Purchase #7	$148.42	3.639%	$153.821	$153.82	$153.80	$153.80
7 purchase totals =	$655.88		$720.6919	$720.69	$720.65	$720.60