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	$29.64	5.499%	$31.2699	$31.27	$31.25	$31.30
Purchase #2	$131.89	5.409%	$139.0239	$139.02	$139.00	$139.00
Purchase #3	$83.29	11.520%	$92.885	$92.89	$92.90	$92.90
Purchase #4	$147.45	8.748%	$160.3489	$160.35	$160.35	$160.30
Purchase #5	$11.71	6.777%	$12.5036	$12.50	$12.50	$12.50
Purchase #6	$99.78	6.718%	$106.4832	$106.48	$106.50	$106.50
Purchase #7	$45.28	7.311%	$48.5904	$48.59	$48.60	$48.60
7 purchase totals =	$549.04		$591.1049	$591.10	$591.10	$591.10