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	$79.99	12.635%	$90.0967	$90.10	$90.10	$90.10
Purchase #2	$89.75	9.719%	$98.4728	$98.47	$98.45	$98.50
Purchase #3	$142.35	10.624%	$157.4733	$157.47	$157.45	$157.50
Purchase #4	$205.61	12.720%	$231.7636	$231.76	$231.75	$231.80
Purchase #5	$243.71	8.799%	$265.154	$265.15	$265.15	$265.20
Purchase #6	$126.81	9.407%	$138.739	$138.74	$138.75	$138.70
Purchase #7	$196.78	11.327%	$219.0693	$219.07	$219.05	$219.10
7 purchase totals =	$1,085.00		$1,200.7687	$1,200.76	$1,200.70	$1,200.90