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	$101.58	6.806%	$108.4935	$108.49	$108.50	$108.50
Purchase #2	$74.41	9.275%	$81.3115	$81.31	$81.30	$81.30
Purchase #3	$181.41	8.492%	$196.8153	$196.82	$196.80	$196.80
Purchase #4	$33.66	6.277%	$35.7728	$35.77	$35.75	$35.80
Purchase #5	$190.92	12.806%	$215.3692	$215.37	$215.35	$215.40
Purchase #6	$188.90	12.491%	$212.4955	$212.50	$212.50	$212.50
Purchase #7	$132.69	5.250%	$139.6562	$139.66	$139.65	$139.70
7 purchase totals =	$903.57		$989.9140	$989.92	$989.85	$990.00