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	$17.71	13.465%	$20.0947	$20.09	$20.10	$20.10
Purchase #2	$80.52	12.267%	$90.3974	$90.40	$90.40	$90.40
Purchase #3	$221.44	10.560%	$244.8241	$244.82	$244.80	$244.80
Purchase #4	$124.84	11.513%	$139.2128	$139.21	$139.20	$139.20
Purchase #5	$201.71	4.624%	$211.0371	$211.04	$211.05	$211.00
Purchase #6	$133.47	6.284%	$141.8573	$141.86	$141.85	$141.90
Purchase #7	$154.46	3.521%	$159.8985	$159.90	$159.90	$159.90
7 purchase totals =	$934.15		$1,007.3219	$1,007.32	$1,007.30	$1,007.30