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	$246.79	13.264%	$279.5242	$279.52	$279.50	$279.50
Purchase #2	$37.81	8.763%	$41.1233	$41.12	$41.10	$41.10
Purchase #3	$130.94	6.618%	$139.6056	$139.61	$139.60	$139.60
Purchase #4	$80.64	12.584%	$90.7877	$90.79	$90.80	$90.80
Purchase #5	$14.89	10.184%	$16.4064	$16.41	$16.40	$16.40
Purchase #6	$97.82	5.499%	$103.1991	$103.20	$103.20	$103.20
Purchase #7	$122.55	13.409%	$138.9827	$138.98	$139.00	$139.00
7 purchase totals =	$731.44		$809.6290	$809.63	$809.60	$809.60