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	$123.80	11.713%	$138.3007	$138.30	$138.30	$138.30
Purchase #2	$108.70	13.608%	$123.4919	$123.49	$123.50	$123.50
Purchase #3	$32.39	4.514%	$33.8521	$33.85	$33.85	$33.90
Purchase #4	$19.71	12.707%	$22.2145	$22.21	$22.20	$22.20
Purchase #5	$175.78	7.771%	$189.4399	$189.44	$189.45	$189.40
Purchase #6	$31.25	13.349%	$35.4216	$35.42	$35.40	$35.40
Purchase #7	$90.92	9.174%	$99.261	$99.26	$99.25	$99.30
7 purchase totals =	$582.55		$641.9817	$641.97	$641.95	$642.00