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	$27.78	6.578%	$29.6074	$29.61	$29.60	$29.60
Purchase #2	$123.67	12.671%	$139.3402	$139.34	$139.35	$139.30
Purchase #3	$173.61	7.790%	$187.1342	$187.13	$187.15	$187.10
Purchase #4	$32.66	8.559%	$35.4554	$35.46	$35.45	$35.50
Purchase #5	$227.43	12.273%	$255.3425	$255.34	$255.35	$255.30
Purchase #6	$95.76	9.598%	$104.951	$104.95	$104.95	$105.00
Purchase #7	$229.72	11.134%	$255.297	$255.30	$255.30	$255.30
7 purchase totals =	$910.63		$1,007.1277	$1,007.13	$1,007.15	$1,007.10