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	$5.92	11.722%	$6.6139	$6.61	$6.60	$6.60
Purchase #2	$241.82	5.298%	$254.6316	$254.63	$254.65	$254.60
Purchase #3	$246.84	6.704%	$263.3882	$263.39	$263.40	$263.40
Purchase #4	$96.62	4.426%	$100.8964	$100.90	$100.90	$100.90
Purchase #5	$248.98	12.278%	$279.5498	$279.55	$279.55	$279.50
Purchase #6	$70.43	9.404%	$77.0532	$77.05	$77.05	$77.10
Purchase #7	$20.64	3.105%	$21.2809	$21.28	$21.30	$21.30
7 purchase totals =	$931.25		$1,003.4140	$1,003.41	$1,003.45	$1,003.40