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	$38.76	3.118%	$39.9685	$39.97	$39.95	$40.00
Purchase #2	$132.31	5.788%	$139.9681	$139.97	$139.95	$140.00
Purchase #3	$29.75	3.665%	$30.8403	$30.84	$30.85	$30.80
Purchase #4	$85.57	5.388%	$90.1805	$90.18	$90.20	$90.20
Purchase #5	$79.89	12.349%	$89.7556	$89.76	$89.75	$89.80
Purchase #6	$216.71	10.246%	$238.9141	$238.91	$238.90	$238.90
Purchase #7	$100.86	9.254%	$110.1936	$110.19	$110.20	$110.20
7 purchase totals =	$683.85		$739.8207	$739.82	$739.80	$739.90