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	$39.86	5.446%	$42.0308	$42.03	$42.05	$42.00
Purchase #2	$84.28	13.712%	$95.8365	$95.84	$95.85	$95.80
Purchase #3	$90.46	6.286%	$96.1463	$96.15	$96.15	$96.10
Purchase #4	$141.38	5.337%	$148.9255	$148.93	$148.95	$148.90
Purchase #5	$8.87	13.656%	$10.0813	$10.08	$10.10	$10.10
Purchase #6	$85.28	7.321%	$91.5233	$91.52	$91.50	$91.50
Purchase #7	$91.83	13.158%	$103.913	$103.91	$103.90	$103.90
7 purchase totals =	$541.96		$588.4567	$588.46	$588.50	$588.30