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	$36.72	7.531%	$39.4854	$39.49	$39.50	$39.50
Purchase #2	$148.49	13.158%	$168.0283	$168.03	$168.05	$168.00
Purchase #3	$18.69	9.244%	$20.4177	$20.42	$20.40	$20.40
Purchase #4	$189.35	13.201%	$214.3461	$214.35	$214.35	$214.30
Purchase #5	$101.95	12.220%	$114.4083	$114.41	$114.40	$114.40
Purchase #6	$143.50	3.297%	$148.2312	$148.23	$148.25	$148.20
Purchase #7	$128.89	13.162%	$145.8545	$145.85	$145.85	$145.90
7 purchase totals =	$767.59		$850.7715	$850.78	$850.80	$850.70