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	$7.78	8.621%	$8.4507	$8.45	$8.45	$8.50
Purchase #2	$249.42	13.278%	$282.538	$282.54	$282.55	$282.50
Purchase #3	$100.55	10.443%	$111.0504	$111.05	$111.05	$111.10
Purchase #4	$177.65	6.638%	$189.4424	$189.44	$189.45	$189.40
Purchase #5	$51.89	5.355%	$54.6687	$54.67	$54.65	$54.70
Purchase #6	$228.33	5.122%	$240.0251	$240.03	$240.05	$240.00
Purchase #7	$39.44	7.558%	$42.4209	$42.42	$42.40	$42.40
7 purchase totals =	$855.06		$928.5962	$928.60	$928.60	$928.60