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	$35.94	8.644%	$39.0467	$39.05	$39.05	$39.00
Purchase #2	$57.66	13.146%	$65.24	$65.24	$65.25	$65.20
Purchase #3	$158.99	5.145%	$167.17	$167.17	$167.15	$167.20
Purchase #4	$130.85	10.742%	$144.9059	$144.91	$144.90	$144.90
Purchase #5	$178.96	4.666%	$187.3103	$187.31	$187.30	$187.30
Purchase #6	$39.57	7.770%	$42.6446	$42.64	$42.65	$42.60
Purchase #7	$30.57	10.687%	$33.837	$33.84	$33.85	$33.80
7 purchase totals =	$632.54		$680.1545	$680.16	$680.15	$680.00