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	$15.97	11.333%	$17.7799	$17.78	$17.80	$17.80
Purchase #2	$65.92	9.260%	$72.0242	$72.02	$72.00	$72.00
Purchase #3	$183.92	13.511%	$208.7694	$208.77	$208.75	$208.80
Purchase #4	$68.81	11.133%	$76.4706	$76.47	$76.45	$76.50
Purchase #5	$67.57	6.308%	$71.8323	$71.83	$71.85	$71.80
Purchase #6	$57.66	7.725%	$62.1142	$62.11	$62.10	$62.10
Purchase #7	$28.65	9.641%	$31.4121	$31.41	$31.40	$31.40
7 purchase totals =	$488.50		$540.4027	$540.39	$540.35	$540.40