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	$123.81	12.386%	$139.1451	$139.15	$139.15	$139.10
Purchase #2	$19.98	4.689%	$20.9169	$20.92	$20.90	$20.90
Purchase #3	$129.53	6.539%	$138	$138.00	$138.00	$138.00
Purchase #4	$28.25	5.693%	$29.8583	$29.86	$29.85	$29.90
Purchase #5	$50.27	3.192%	$51.8746	$51.87	$51.85	$51.90
Purchase #6	$92.43	7.539%	$99.3983	$99.40	$99.40	$99.40
Purchase #7	$146.73	9.445%	$160.5886	$160.59	$160.60	$160.60
7 purchase totals =	$591.00		$639.7818	$639.79	$639.75	$639.80