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	$72.84	10.195%	$80.266	$80.27	$80.25	$80.30
Purchase #2	$87.80	11.408%	$97.8162	$97.82	$97.80	$97.80
Purchase #3	$6.87	7.253%	$7.3683	$7.37	$7.35	$7.40
Purchase #4	$106.56	9.625%	$116.8164	$116.82	$116.80	$116.80
Purchase #5	$8.86	7.771%	$9.5485	$9.55	$9.55	$9.50
Purchase #6	$104.73	6.845%	$111.8988	$111.90	$111.90	$111.90
Purchase #7	$148.58	5.583%	$156.8752	$156.88	$156.90	$156.90
7 purchase totals =	$536.24		$580.5894	$580.61	$580.55	$580.60