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	$18.96	12.473%	$21.3249	$21.32	$21.30	$21.30
Purchase #2	$150.30	4.258%	$156.6998	$156.70	$156.70	$156.70
Purchase #3	$121.76	9.535%	$133.3698	$133.37	$133.35	$133.40
Purchase #4	$11.56	13.719%	$13.1459	$13.15	$13.15	$13.10
Purchase #5	$127.57	10.612%	$141.1077	$141.11	$141.10	$141.10
Purchase #6	$18.31	4.472%	$19.1288	$19.13	$19.15	$19.10
Purchase #7	$46.96	10.334%	$51.8128	$51.81	$51.80	$51.80
7 purchase totals =	$495.42		$536.5897	$536.59	$536.55	$536.50