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	$40.26	5.497%	$42.4731	$42.47	$42.45	$42.50
Purchase #2	$116.97	9.678%	$128.2904	$128.29	$128.30	$128.30
Purchase #3	$99.93	10.126%	$110.0489	$110.05	$110.05	$110.00
Purchase #4	$40.67	6.835%	$43.4498	$43.45	$43.45	$43.40
Purchase #5	$164.73	4.424%	$172.0177	$172.02	$172.00	$172.00
Purchase #6	$72.56	13.117%	$82.0777	$82.08	$82.10	$82.10
Purchase #7	$18.84	8.170%	$20.3792	$20.38	$20.40	$20.40
7 purchase totals =	$553.96		$598.7368	$598.74	$598.75	$598.70