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	$122.95	13.280%	$139.2778	$139.28	$139.30	$139.30
Purchase #2	$250.46	13.451%	$284.1494	$284.15	$284.15	$284.10
Purchase #3	$185.57	10.729%	$205.4798	$205.48	$205.50	$205.50
Purchase #4	$95.87	9.849%	$105.3122	$105.31	$105.30	$105.30
Purchase #5	$35.89	6.807%	$38.333	$38.33	$38.35	$38.30
Purchase #6	$131.70	9.481%	$144.1865	$144.19	$144.20	$144.20
Purchase #7	$58.47	3.606%	$60.5784	$60.58	$60.60	$60.60
7 purchase totals =	$880.91		$977.3171	$977.32	$977.40	$977.30