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	$21.52	13.123%	$24.3441	$24.34	$24.35	$24.30
Purchase #2	$249.43	12.869%	$281.5291	$281.53	$281.55	$281.50
Purchase #3	$112.66	13.474%	$127.8398	$127.84	$127.85	$127.80
Purchase #4	$184.31	3.241%	$190.2835	$190.28	$190.30	$190.30
Purchase #5	$5.81	13.235%	$6.579	$6.58	$6.60	$6.60
Purchase #6	$101.70	11.144%	$113.0334	$113.03	$113.05	$113.00
Purchase #7	$74.48	5.127%	$78.2986	$78.30	$78.30	$78.30
7 purchase totals =	$749.91		$821.9075	$821.90	$822.00	$821.80