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	$45.78	6.803%	$48.8944	$48.89	$48.90	$48.90
Purchase #2	$128.60	12.706%	$144.9399	$144.94	$144.95	$144.90
Purchase #3	$43.60	6.187%	$46.2975	$46.30	$46.30	$46.30
Purchase #4	$93.31	10.619%	$103.2186	$103.22	$103.20	$103.20
Purchase #5	$122.69	13.706%	$139.5059	$139.51	$139.50	$139.50
Purchase #6	$46.51	5.218%	$48.9369	$48.94	$48.95	$48.90
Purchase #7	$115.71	10.169%	$127.4765	$127.48	$127.50	$127.50
7 purchase totals =	$596.20		$659.2697	$659.28	$659.30	$659.20