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	$15.99	11.159%	$17.7743	$17.77	$17.75	$17.80
Purchase #2	$50.30	7.160%	$53.9015	$53.90	$53.90	$53.90
Purchase #3	$117.78	9.143%	$128.5486	$128.55	$128.55	$128.50
Purchase #4	$97.82	13.584%	$111.1079	$111.11	$111.10	$111.10
Purchase #5	$87.81	11.329%	$97.758	$97.76	$97.75	$97.80
Purchase #6	$91.40	10.469%	$100.9687	$100.97	$100.95	$101.00
Purchase #7	$123.97	7.180%	$132.871	$132.87	$132.85	$132.90
7 purchase totals =	$585.07		$642.9300	$642.93	$642.85	$643.00