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	$12.44	5.827%	$13.1649	$13.16	$13.15	$13.20
Purchase #2	$145.64	13.870%	$165.8403	$165.84	$165.85	$165.80
Purchase #3	$235.51	8.389%	$255.2669	$255.27	$255.25	$255.30
Purchase #4	$104.38	10.614%	$115.4589	$115.46	$115.45	$115.50
Purchase #5	$38.85	8.633%	$42.2039	$42.20	$42.20	$42.20
Purchase #6	$118.38	7.836%	$127.6563	$127.66	$127.65	$127.70
Purchase #7	$239.58	8.363%	$259.6161	$259.62	$259.60	$259.60
7 purchase totals =	$894.78		$979.2073	$979.21	$979.15	$979.30