If the spars are badly warped, replace them (Or email GP and ask THEM to replace them).

But if they aren't off by much, don't worry about it. As long as everything is held straight with pins and/or weights, once the shear webs and sheeting are in place, the wing will hold it's shape.

Any minor variences can be straightened by shrinking the covering.

Besides, in a Trainer, it's really not that critical.