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	$122.92	10.475%	$135.7959	$135.80	$135.80	$135.80
Purchase #2	$37.96	4.733%	$39.7566	$39.76	$39.75	$39.80
Purchase #3	$232.88	4.455%	$243.2548	$243.25	$243.25	$243.30
Purchase #4	$242.45	5.406%	$255.5568	$255.56	$255.55	$255.60
Purchase #5	$37.45	12.257%	$42.0402	$42.04	$42.05	$42.00
Purchase #6	$108.45	3.794%	$112.5646	$112.56	$112.55	$112.60
Purchase #7	$9.61	7.274%	$10.309	$10.31	$10.30	$10.30
7 purchase totals =	$791.72		$839.2779	$839.28	$839.25	$839.40