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	$16.65	9.860%	$18.2917	$18.29	$18.30	$18.30
Purchase #2	$115.72	5.834%	$122.4711	$122.47	$122.45	$122.50
Purchase #3	$101.52	7.786%	$109.4243	$109.42	$109.40	$109.40
Purchase #4	$109.77	4.126%	$114.2991	$114.30	$114.30	$114.30
Purchase #5	$103.63	12.479%	$116.562	$116.56	$116.55	$116.60
Purchase #6	$220.61	11.768%	$246.5714	$246.57	$246.55	$246.60
Purchase #7	$103.51	7.257%	$111.0217	$111.02	$111.00	$111.00
7 purchase totals =	$771.41		$838.6413	$838.63	$838.55	$838.70