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	$79.83	12.630%	$89.9125	$89.91	$89.90	$89.90
Purchase #2	$37.89	11.417%	$42.2159	$42.22	$42.20	$42.20
Purchase #3	$235.66	9.715%	$258.5544	$258.55	$258.55	$258.60
Purchase #4	$190.65	13.190%	$215.7967	$215.80	$215.80	$215.80
Purchase #5	$157.89	12.582%	$177.7557	$177.76	$177.75	$177.80
Purchase #6	$142.33	11.513%	$158.7165	$158.72	$158.70	$158.70
Purchase #7	$226.50	13.422%	$256.9008	$256.90	$256.90	$256.90
7 purchase totals =	$1,070.75		$1,199.8525	$1,199.86	$1,199.80	$1,199.90