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	$36.25	7.520%	$38.976	$38.98	$39.00	$39.00
Purchase #2	$99.29	6.171%	$105.4172	$105.42	$105.40	$105.40
Purchase #3	$248.26	9.749%	$272.4629	$272.46	$272.45	$272.50
Purchase #4	$36.30	12.647%	$40.8909	$40.89	$40.90	$40.90
Purchase #5	$212.77	4.491%	$222.3255	$222.33	$222.35	$222.30
Purchase #6	$109.98	12.457%	$123.6802	$123.68	$123.70	$123.70
Purchase #7	$17.50	3.535%	$18.1186	$18.12	$18.10	$18.10
7 purchase totals =	$760.35		$821.8713	$821.88	$821.90	$821.90