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	$19.66	7.530%	$21.1404	$21.14	$21.15	$21.10
Purchase #2	$107.51	4.618%	$112.4748	$112.47	$112.45	$112.50
Purchase #3	$105.27	9.118%	$114.8685	$114.87	$114.85	$114.90
Purchase #4	$126.95	4.229%	$132.3187	$132.32	$132.30	$132.30
Purchase #5	$141.69	12.140%	$158.8912	$158.89	$158.90	$158.90
Purchase #6	$100.50	8.307%	$108.8485	$108.85	$108.85	$108.80
Purchase #7	$127.33	13.814%	$144.9194	$144.92	$144.90	$144.90
7 purchase totals =	$728.91		$793.4615	$793.46	$793.40	$793.40