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	$14.92	5.832%	$15.7901	$15.79	$15.80	$15.80
Purchase #2	$123.76	11.359%	$137.8179	$137.82	$137.80	$137.80
Purchase #3	$62.39	3.325%	$64.4645	$64.46	$64.45	$64.50
Purchase #4	$19.92	4.454%	$20.8072	$20.81	$20.80	$20.80
Purchase #5	$230.32	7.847%	$248.3932	$248.39	$248.40	$248.40
Purchase #6	$140.96	10.695%	$156.0357	$156.04	$156.05	$156.00
Purchase #7	$6.62	3.579%	$6.8569	$6.86	$6.85	$6.90
7 purchase totals =	$598.89		$650.1655	$650.17	$650.15	$650.20