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	$16.70	12.777%	$18.8338	$18.83	$18.85	$18.80
Purchase #2	$199.73	4.399%	$208.5161	$208.52	$208.50	$208.50
Purchase #3	$83.64	8.224%	$90.5186	$90.52	$90.50	$90.50
Purchase #4	$121.37	6.605%	$129.3865	$129.39	$129.40	$129.40
Purchase #5	$20.99	3.778%	$21.783	$21.78	$21.80	$21.80
Purchase #6	$95.59	8.135%	$103.3662	$103.37	$103.35	$103.40
Purchase #7	$87.84	8.548%	$95.3486	$95.35	$95.35	$95.30
7 purchase totals =	$625.86		$667.7528	$667.76	$667.75	$667.70