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	$39.74	6.146%	$42.1824	$42.18	$42.20	$42.20
Purchase #2	$128.86	12.552%	$145.0345	$145.03	$145.05	$145.00
Purchase #3	$110.37	8.809%	$120.0925	$120.09	$120.10	$120.10
Purchase #4	$111.45	13.551%	$126.5526	$126.55	$126.55	$126.60
Purchase #5	$57.36	9.725%	$62.9383	$62.94	$62.95	$62.90
Purchase #6	$240.61	8.629%	$261.3722	$261.37	$261.35	$261.40
Purchase #7	$93.25	7.693%	$100.4237	$100.42	$100.40	$100.40
7 purchase totals =	$781.64		$858.5962	$858.58	$858.60	$858.60