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	$24.42	12.300%	$27.4237	$27.42	$27.40	$27.40
Purchase #2	$32.67	6.696%	$34.8576	$34.86	$34.85	$34.90
Purchase #3	$152.90	9.575%	$167.5402	$167.54	$167.55	$167.50
Purchase #4	$22.75	9.160%	$24.8339	$24.83	$24.85	$24.80
Purchase #5	$87.65	13.177%	$99.1996	$99.20	$99.20	$99.20
Purchase #6	$172.61	4.608%	$180.5639	$180.56	$180.55	$180.60
Purchase #7	$93.42	11.440%	$104.1072	$104.11	$104.10	$104.10
7 purchase totals =	$586.42		$638.5261	$638.52	$638.50	$638.50