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	$72.42	10.476%	$80.0067	$80.01	$80.00	$80.00
Purchase #2	$33.97	12.435%	$38.1942	$38.19	$38.20	$38.20
Purchase #3	$129.31	11.138%	$143.7125	$143.71	$143.70	$143.70
Purchase #4	$24.90	10.240%	$27.4498	$27.45	$27.45	$27.40
Purchase #5	$99.95	12.699%	$112.6427	$112.64	$112.65	$112.60
Purchase #6	$72.95	10.316%	$80.4755	$80.48	$80.50	$80.50
Purchase #7	$20.47	9.117%	$22.3362	$22.34	$22.35	$22.30
7 purchase totals =	$453.97		$504.8176	$504.82	$504.85	$504.70