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	$6.75	4.107%	$7.0272	$7.03	$7.05	$7.00
Purchase #2	$73.67	13.733%	$83.7871	$83.79	$83.80	$83.80
Purchase #3	$77.61	10.726%	$85.9344	$85.93	$85.95	$85.90
Purchase #4	$62.32	12.357%	$70.0209	$70.02	$70.00	$70.00
Purchase #5	$6.84	11.719%	$7.6416	$7.64	$7.65	$7.60
Purchase #6	$186.35	10.457%	$205.8366	$205.84	$205.85	$205.80
Purchase #7	$172.67	3.742%	$179.1313	$179.13	$179.15	$179.10
7 purchase totals =	$586.21		$639.3791	$639.38	$639.45	$639.20