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	$5.72	12.866%	$6.4559	$6.46	$6.45	$6.50
Purchase #2	$98.53	9.778%	$108.1643	$108.16	$108.15	$108.20
Purchase #3	$197.98	3.482%	$204.8737	$204.87	$204.85	$204.90
Purchase #4	$89.50	12.625%	$100.7994	$100.80	$100.80	$100.80
Purchase #5	$79.85	13.616%	$90.7224	$90.72	$90.70	$90.70
Purchase #6	$12.46	8.249%	$13.4878	$13.49	$13.50	$13.50
Purchase #7	$109.39	12.498%	$123.0616	$123.06	$123.05	$123.10
7 purchase totals =	$593.43		$647.5651	$647.56	$647.50	$647.70