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	$26.98	11.418%	$30.0606	$30.06	$30.05	$30.10
Purchase #2	$173.75	9.766%	$190.7184	$190.72	$190.70	$190.70
Purchase #3	$171.36	4.349%	$178.8124	$178.81	$178.80	$178.80
Purchase #4	$34.95	12.510%	$39.3222	$39.32	$39.30	$39.30
Purchase #5	$165.25	5.588%	$174.4842	$174.48	$174.50	$174.50
Purchase #6	$37.91	12.364%	$42.5972	$42.60	$42.60	$42.60
Purchase #7	$170.89	7.613%	$183.8999	$183.90	$183.90	$183.90
7 purchase totals =	$781.09		$839.8949	$839.89	$839.85	$839.90