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	$140.29	13.366%	$159.0412	$159.04	$159.05	$159.00
Purchase #2	$161.64	4.694%	$169.2274	$169.23	$169.25	$169.20
Purchase #3	$237.52	9.366%	$259.7661	$259.77	$259.75	$259.80
Purchase #4	$28.25	4.664%	$29.5676	$29.57	$29.55	$29.60
Purchase #5	$90.82	12.122%	$101.8292	$101.83	$101.85	$101.80
Purchase #6	$53.59	5.498%	$56.5364	$56.54	$56.55	$56.50
Purchase #7	$12.55	10.583%	$13.8782	$13.88	$13.90	$13.90
7 purchase totals =	$724.66		$789.8461	$789.86	$789.90	$789.80