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	$15.39	3.340%	$15.904	$15.90	$15.90	$15.90
Purchase #2	$235.48	3.752%	$244.3152	$244.32	$244.30	$244.30
Purchase #3	$195.38	5.357%	$205.8465	$205.85	$205.85	$205.80
Purchase #4	$102.91	8.429%	$111.5843	$111.58	$111.60	$111.60
Purchase #5	$134.37	9.849%	$147.6041	$147.60	$147.60	$147.60
Purchase #6	$122.56	6.439%	$130.4516	$130.45	$130.45	$130.50
Purchase #7	$80.30	4.813%	$84.1648	$84.16	$84.15	$84.20
7 purchase totals =	$886.39		$939.8705	$939.86	$939.85	$939.90