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	$15.60	10.852%	$17.2929	$17.29	$17.30	$17.30
Purchase #2	$224.48	6.391%	$238.8265	$238.83	$238.85	$238.80
Purchase #3	$123.89	12.479%	$139.3502	$139.35	$139.35	$139.40
Purchase #4	$22.46	13.522%	$25.497	$25.50	$25.50	$25.50
Purchase #5	$110.72	5.122%	$116.3911	$116.39	$116.40	$116.40
Purchase #6	$107.94	3.657%	$111.8874	$111.89	$111.90	$111.90
Purchase #7	$144.67	3.473%	$149.6944	$149.69	$149.70	$149.70
7 purchase totals =	$749.76		$798.9395	$798.94	$799.00	$799.00