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	$39.60	6.741%	$42.2694	$42.27	$42.25	$42.30
Purchase #2	$6.47	10.288%	$7.1356	$7.14	$7.15	$7.10
Purchase #3	$239.59	11.368%	$266.8266	$266.83	$266.85	$266.80
Purchase #4	$50.51	7.750%	$54.4245	$54.42	$54.40	$54.40
Purchase #5	$16.70	9.498%	$18.2862	$18.29	$18.30	$18.30
Purchase #6	$104.55	11.670%	$116.751	$116.75	$116.75	$116.80
Purchase #7	$33.74	6.239%	$35.845	$35.85	$35.85	$35.80
7 purchase totals =	$491.16		$541.5383	$541.55	$541.55	$541.50