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	$39.89	7.145%	$42.7401	$42.74	$42.75	$42.70
Purchase #2	$99.42	12.455%	$111.8028	$111.80	$111.80	$111.80
Purchase #3	$85.70	8.547%	$93.0248	$93.02	$93.00	$93.00
Purchase #4	$145.93	9.194%	$159.3468	$159.35	$159.35	$159.30
Purchase #5	$156.27	13.168%	$176.8476	$176.85	$176.85	$176.80
Purchase #6	$80.96	10.757%	$89.6689	$89.67	$89.65	$89.70
Purchase #7	$32.98	11.676%	$36.8307	$36.83	$36.85	$36.80
7 purchase totals =	$641.15		$710.2617	$710.26	$710.25	$710.10