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	$37.75	5.861%	$39.9625	$39.96	$39.95	$40.00
Purchase #2	$37.55	13.461%	$42.6046	$42.60	$42.60	$42.60
Purchase #3	$152.49	4.789%	$159.7927	$159.79	$159.80	$159.80
Purchase #4	$56.26	6.216%	$59.7571	$59.76	$59.75	$59.80
Purchase #5	$235.70	8.819%	$256.4864	$256.49	$256.50	$256.50
Purchase #6	$135.56	13.616%	$154.0178	$154.02	$154.00	$154.00
Purchase #7	$108.55	7.333%	$116.51	$116.51	$116.50	$116.50
7 purchase totals =	$763.86		$829.1311	$829.13	$829.10	$829.20