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	$55.82	3.402%	$57.719	$57.72	$57.70	$57.70
Purchase #2	$111.81	10.136%	$123.1431	$123.14	$123.15	$123.10
Purchase #3	$6.88	11.533%	$7.6735	$7.67	$7.65	$7.70
Purchase #4	$239.72	5.662%	$253.2929	$253.29	$253.30	$253.30
Purchase #5	$180.92	8.585%	$196.452	$196.45	$196.45	$196.50
Purchase #6	$100.52	8.252%	$108.8149	$108.81	$108.80	$108.80
Purchase #7	$82.38	11.520%	$91.8702	$91.87	$91.85	$91.90
7 purchase totals =	$778.05		$838.9656	$838.95	$838.90	$839.00