I have tried to straighten wings using the water, warp and dry method. The problem I have had is the wing wants to go back into the warped shape over time. Some time it takes a month or two. If you use this method, wet all the sheeting: leading and trailing edge, top and bottom. Then bend the wing past straight so it is warped a little in the opposite direction. Let it dry overnight. It will spring back over time and hopefully end up about straight.

P.S. - I have heard that adding ammonia to the water is of no benefit. It also stinks and discolors the balsa.