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	$13.81	12.227%	$15.4985	$15.50	$15.50	$15.50
Purchase #2	$140.30	8.857%	$152.7264	$152.73	$152.75	$152.70
Purchase #3	$80.44	12.301%	$90.3349	$90.33	$90.35	$90.30
Purchase #4	$12.33	4.154%	$12.8422	$12.84	$12.85	$12.80
Purchase #5	$150.77	5.607%	$159.2237	$159.22	$159.20	$159.20
Purchase #6	$82.68	9.429%	$90.4759	$90.48	$90.50	$90.50
Purchase #7	$27.45	8.202%	$29.7014	$29.70	$29.70	$29.70
7 purchase totals =	$507.78		$550.8030	$550.80	$550.85	$550.70