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	$21.79	12.485%	$24.5105	$24.51	$24.50	$24.50
Purchase #2	$128.87	4.692%	$134.9166	$134.92	$134.90	$134.90
Purchase #3	$73.82	5.766%	$78.0765	$78.08	$78.10	$78.10
Purchase #4	$102.47	8.489%	$111.1687	$111.17	$111.15	$111.20
Purchase #5	$81.25	11.748%	$90.7953	$90.80	$90.80	$90.80
Purchase #6	$50.66	12.569%	$57.0275	$57.03	$57.05	$57.00
Purchase #7	$161.84	4.328%	$168.8444	$168.84	$168.85	$168.80
7 purchase totals =	$620.70		$665.3395	$665.35	$665.35	$665.30