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	$16.41	12.795%	$18.5097	$18.51	$18.50	$18.50
Purchase #2	$196.46	8.697%	$213.5461	$213.55	$213.55	$213.50
Purchase #3	$201.76	10.836%	$223.6227	$223.62	$223.60	$223.60
Purchase #4	$90.71	13.709%	$103.1454	$103.15	$103.15	$103.10
Purchase #5	$51.70	11.838%	$57.8202	$57.82	$57.80	$57.80
Purchase #6	$61.65	11.346%	$68.6448	$68.64	$68.65	$68.60
Purchase #7	$139.83	9.861%	$153.6186	$153.62	$153.60	$153.60
7 purchase totals =	$758.52		$838.9075	$838.91	$838.85	$838.70