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	$74.59	7.240%	$79.9903	$79.99	$80.00	$80.00
Purchase #2	$121.31	11.647%	$135.439	$135.44	$135.45	$135.40
Purchase #3	$44.77	13.731%	$50.9174	$50.92	$50.90	$50.90
Purchase #4	$43.78	6.139%	$46.4677	$46.47	$46.45	$46.50
Purchase #5	$197.63	12.730%	$222.7883	$222.79	$222.80	$222.80
Purchase #6	$27.27	8.153%	$29.4933	$29.49	$29.50	$29.50
Purchase #7	$91.56	10.228%	$100.9248	$100.92	$100.90	$100.90
7 purchase totals =	$600.91		$666.0208	$666.02	$666.00	$666.00