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	$51.68	5.425%	$54.4836	$54.48	$54.50	$54.50
Purchase #2	$191.34	13.688%	$217.5306	$217.53	$217.55	$217.50
Purchase #3	$36.89	10.200%	$40.6528	$40.65	$40.65	$40.70
Purchase #4	$43.70	3.269%	$45.1286	$45.13	$45.15	$45.10
Purchase #5	$60.58	6.503%	$64.5195	$64.52	$64.50	$64.50
Purchase #6	$40.47	3.317%	$41.8124	$41.81	$41.80	$41.80
Purchase #7	$163.42	7.255%	$175.2761	$175.28	$175.30	$175.30
7 purchase totals =	$588.08		$639.4036	$639.40	$639.45	$639.40