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	$39.48	6.410%	$42.0107	$42.01	$42.00	$42.00
Purchase #2	$150.47	5.812%	$159.2153	$159.22	$159.20	$159.20
Purchase #3	$134.42	7.751%	$144.8389	$144.84	$144.85	$144.80
Purchase #4	$135.39	5.249%	$142.4966	$142.50	$142.50	$142.50
Purchase #5	$100.88	11.314%	$112.2936	$112.29	$112.30	$112.30
Purchase #6	$7.76	7.196%	$8.3184	$8.32	$8.30	$8.30
Purchase #7	$44.43	13.479%	$50.4187	$50.42	$50.40	$50.40
7 purchase totals =	$612.83		$659.5922	$659.60	$659.55	$659.50