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	$18.91	7.430%	$20.315	$20.32	$20.30	$20.30
Purchase #2	$175.50	4.496%	$183.3905	$183.39	$183.40	$183.40
Purchase #3	$86.61	6.201%	$91.9807	$91.98	$92.00	$92.00
Purchase #4	$12.32	12.507%	$13.8609	$13.86	$13.85	$13.90
Purchase #5	$84.42	6.763%	$90.1293	$90.13	$90.15	$90.10
Purchase #6	$133.88	7.366%	$143.7416	$143.74	$143.75	$143.70
Purchase #7	$35.95	3.696%	$37.2787	$37.28	$37.30	$37.30
7 purchase totals =	$547.59		$580.6967	$580.70	$580.75	$580.70