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	$18.91	13.338%	$21.4322	$21.43	$21.45	$21.40
Purchase #2	$64.45	6.189%	$68.4388	$68.44	$68.45	$68.40
Purchase #3	$12.48	7.625%	$13.4316	$13.43	$13.45	$13.40
Purchase #4	$99.93	3.843%	$103.7703	$103.77	$103.75	$103.80
Purchase #5	$103.57	11.439%	$115.4174	$115.42	$115.40	$115.40
Purchase #6	$81.28	11.866%	$90.9247	$90.92	$90.90	$90.90
Purchase #7	$191.31	11.707%	$213.7067	$213.71	$213.70	$213.70
7 purchase totals =	$571.93		$627.1217	$627.12	$627.10	$627.00