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	$12.42	4.595%	$12.9907	$12.99	$13.00	$13.00
Purchase #2	$237.59	12.493%	$267.2721	$267.27	$267.25	$267.30
Purchase #3	$115.30	11.426%	$128.4742	$128.47	$128.45	$128.50
Purchase #4	$173.34	13.250%	$196.3076	$196.31	$196.30	$196.30
Purchase #5	$237.43	7.571%	$255.4058	$255.41	$255.40	$255.40
Purchase #6	$99.88	10.311%	$110.1786	$110.18	$110.20	$110.20
Purchase #7	$107.83	4.134%	$112.2877	$112.29	$112.30	$112.30
7 purchase totals =	$983.79		$1,082.9167	$1,082.92	$1,082.90	$1,083.00