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	$248.96	12.619%	$280.3763	$280.38	$280.40	$280.40
Purchase #2	$99.67	13.426%	$113.0517	$113.05	$113.05	$113.10
Purchase #3	$62.75	7.253%	$67.3013	$67.30	$67.30	$67.30
Purchase #4	$77.82	13.710%	$88.4891	$88.49	$88.50	$88.50
Purchase #5	$158.46	13.776%	$180.2894	$180.29	$180.30	$180.30
Purchase #6	$102.40	3.745%	$106.2349	$106.23	$106.25	$106.20
Purchase #7	$19.31	4.725%	$20.2224	$20.22	$20.20	$20.20
7 purchase totals =	$769.37		$855.9651	$855.96	$856.00	$856.00