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	$74.78	7.159%	$80.1335	$80.13	$80.15	$80.10
Purchase #2	$81.32	11.551%	$90.7133	$90.71	$90.70	$90.70
Purchase #3	$69.40	12.872%	$78.3332	$78.33	$78.35	$78.30
Purchase #4	$151.37	6.764%	$161.6087	$161.61	$161.60	$161.60
Purchase #5	$18.85	5.745%	$19.9329	$19.93	$19.95	$19.90
Purchase #6	$120.29	6.721%	$128.3747	$128.37	$128.35	$128.40
Purchase #7	$75.57	7.673%	$81.3685	$81.37	$81.35	$81.40
7 purchase totals =	$591.58		$640.4648	$640.45	$640.45	$640.40