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	$13.90	10.147%	$15.3104	$15.31	$15.30	$15.30
Purchase #2	$137.65	9.572%	$150.8259	$150.83	$150.85	$150.80
Purchase #3	$50.76	8.243%	$54.9441	$54.94	$54.95	$54.90
Purchase #4	$102.48	12.111%	$114.8914	$114.89	$114.90	$114.90
Purchase #5	$230.51	10.165%	$253.9413	$253.94	$253.95	$253.90
Purchase #6	$76.85	8.177%	$83.134	$83.13	$83.15	$83.10
Purchase #7	$125.43	9.435%	$137.2643	$137.26	$137.25	$137.30
7 purchase totals =	$737.58		$810.3114	$810.30	$810.35	$810.20