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	$112.53	13.736%	$127.9871	$127.99	$128.00	$128.00
Purchase #2	$64.77	8.665%	$70.3823	$70.38	$70.40	$70.40
Purchase #3	$228.40	5.304%	$240.5143	$240.51	$240.50	$240.50
Purchase #4	$129.89	7.144%	$139.1693	$139.17	$139.15	$139.20
Purchase #5	$149.32	11.156%	$165.9781	$165.98	$166.00	$166.00
Purchase #6	$111.53	10.588%	$123.3388	$123.34	$123.35	$123.30
Purchase #7	$22.61	3.159%	$23.3242	$23.32	$23.30	$23.30
7 purchase totals =	$819.05		$890.6941	$890.69	$890.70	$890.70