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	$75.76	3.482%	$78.398	$78.40	$78.40	$78.40
Purchase #2	$95.73	8.202%	$103.5818	$103.58	$103.60	$103.60
Purchase #3	$48.41	4.133%	$50.4108	$50.41	$50.40	$50.40
Purchase #4	$57.83	6.436%	$61.5519	$61.55	$61.55	$61.60
Purchase #5	$81.77	4.592%	$85.5249	$85.52	$85.50	$85.50
Purchase #6	$110.75	13.593%	$125.8042	$125.80	$125.80	$125.80
Purchase #7	$129.41	5.613%	$136.6738	$136.67	$136.65	$136.70
7 purchase totals =	$599.66		$641.9454	$641.93	$641.90	$642.00