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	$18.92	7.794%	$20.3946	$20.39	$20.40	$20.40
Purchase #2	$27.32	8.194%	$29.5586	$29.56	$29.55	$29.60
Purchase #3	$111.39	11.135%	$123.7933	$123.79	$123.80	$123.80
Purchase #4	$36.42	7.856%	$39.2812	$39.28	$39.30	$39.30
Purchase #5	$87.27	5.144%	$91.7592	$91.76	$91.75	$91.80
Purchase #6	$124.49	5.550%	$131.3992	$131.40	$131.40	$131.40
Purchase #7	$100.83	4.132%	$104.9963	$105.00	$105.00	$105.00
7 purchase totals =	$506.64		$541.1824	$541.18	$541.20	$541.30