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	$16.51	13.632%	$18.7606	$18.76	$18.75	$18.80
Purchase #2	$78.81	7.712%	$84.8878	$84.89	$84.90	$84.90
Purchase #3	$204.45	10.550%	$226.0195	$226.02	$226.00	$226.00
Purchase #4	$160.79	12.743%	$181.2795	$181.28	$181.30	$181.30
Purchase #5	$165.78	8.580%	$180.0039	$180.00	$180.00	$180.00
Purchase #6	$82.29	8.809%	$89.5389	$89.54	$89.55	$89.50
Purchase #7	$88.83	13.266%	$100.6142	$100.61	$100.60	$100.60
7 purchase totals =	$797.46		$881.1044	$881.10	$881.10	$881.10