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	$16.76	10.409%	$18.5045	$18.50	$18.50	$18.50
Purchase #2	$182.28	9.134%	$198.9295	$198.93	$198.95	$198.90
Purchase #3	$220.93	9.868%	$242.7314	$242.73	$242.75	$242.70
Purchase #4	$90.58	11.256%	$100.7757	$100.78	$100.80	$100.80
Purchase #5	$131.84	9.727%	$144.6641	$144.66	$144.65	$144.70
Purchase #6	$188.46	5.139%	$198.145	$198.15	$198.15	$198.10
Purchase #7	$153.90	11.840%	$172.1218	$172.12	$172.10	$172.10
7 purchase totals =	$984.75		$1,075.8720	$1,075.87	$1,075.90	$1,075.80