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	$38.60	3.583%	$39.983	$39.98	$40.00	$40.00
Purchase #2	$141.48	13.509%	$160.5925	$160.59	$160.60	$160.60
Purchase #3	$121.69	8.577%	$132.1274	$132.13	$132.15	$132.10
Purchase #4	$96.35	9.148%	$105.1641	$105.16	$105.15	$105.20
Purchase #5	$242.25	9.412%	$265.0506	$265.05	$265.05	$265.10
Purchase #6	$37.91	9.349%	$41.4542	$41.45	$41.45	$41.50
Purchase #7	$147.31	8.355%	$159.6178	$159.62	$159.60	$159.60
7 purchase totals =	$825.59		$903.9896	$903.98	$904.00	$904.10