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	$13.72	12.114%	$15.382	$15.38	$15.40	$15.40
Purchase #2	$131.40	13.256%	$148.8184	$148.82	$148.80	$148.80
Purchase #3	$76.76	11.810%	$85.8254	$85.83	$85.85	$85.80
Purchase #4	$246.92	3.521%	$255.6141	$255.61	$255.60	$255.60
Purchase #5	$67.90	11.435%	$75.6644	$75.66	$75.65	$75.70
Purchase #6	$188.99	6.116%	$200.5486	$200.55	$200.55	$200.50
Purchase #7	$139.52	7.111%	$149.4413	$149.44	$149.45	$149.40
7 purchase totals =	$865.21		$931.2942	$931.29	$931.30	$931.20