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	$27.76	11.359%	$30.9133	$30.91	$30.90	$30.90
Purchase #2	$153.39	3.537%	$158.8154	$158.82	$158.80	$158.80
Purchase #3	$144.41	6.307%	$153.5179	$153.52	$153.50	$153.50
Purchase #4	$116.93	9.665%	$128.2313	$128.23	$128.25	$128.20
Purchase #5	$143.87	3.232%	$148.5199	$148.52	$148.50	$148.50
Purchase #6	$219.33	13.279%	$248.4548	$248.45	$248.45	$248.50
Purchase #7	$178.77	5.637%	$188.8473	$188.85	$188.85	$188.80
7 purchase totals =	$984.46		$1,057.2999	$1,057.30	$1,057.25	$1,057.20