You can set the washout with covering...the wing is flexible enough. After it's covered, twist the wing a bit to add the washout and use a heat gun to shrink the covering. You may have to twist a little bit more than the needed amount because of springback. This will hold for a number of flying sessions.

If your wing has zero or a small amount of washout, you'll be fine. Make sure it's the same for both left and right wing halves. If you have the opposite of washout, washin, then you HAVE to do something about it. Washin is rarely desireable in RC models.