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	$18.84	12.377%	$21.1718	$21.17	$21.15	$21.20
Purchase #2	$91.51	10.212%	$100.855	$100.86	$100.85	$100.90
Purchase #3	$29.65	9.418%	$32.4424	$32.44	$32.45	$32.40
Purchase #4	$13.50	10.760%	$14.9526	$14.95	$14.95	$15.00
Purchase #5	$52.58	12.698%	$59.2566	$59.26	$59.25	$59.30
Purchase #6	$248.63	3.644%	$257.6901	$257.69	$257.70	$257.70
Purchase #7	$177.75	5.245%	$187.073	$187.07	$187.05	$187.10
7 purchase totals =	$632.46		$673.4415	$673.44	$673.40	$673.60