I'd suggest placing weight on each wing panel to hold the wing panels down on the jigs while gluing the dihedral braces, and also while you apply the top center wing sheeting. Using weight makes it easier than pinning to make any necessary adjustments. I like to use a 1 liter drinking water bottle on each panel. With these you can vary the amount of weight you apply with the amount of water inside them, being careful to not distort the panels.
Yes, you can pull a warp out with the covering, but I think everyone would agree that it's much better to start with a straight and true airframe.