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	$20.88	11.187%	$23.2158	$23.22	$23.20	$23.20
Purchase #2	$98.63	5.393%	$103.9491	$103.95	$103.95	$103.90
Purchase #3	$36.45	10.527%	$40.2871	$40.29	$40.30	$40.30
Purchase #4	$250.40	12.202%	$280.9538	$280.95	$280.95	$281.00
Purchase #5	$48.77	7.606%	$52.4794	$52.48	$52.50	$52.50
Purchase #6	$89.50	6.392%	$95.2208	$95.22	$95.20	$95.20
Purchase #7	$40.25	6.600%	$42.9065	$42.91	$42.90	$42.90
7 purchase totals =	$584.88		$639.0125	$639.02	$639.00	$639.00