Warping:

Foam cores are not strong, in and of themselves.  Cutting the airfoils in foam blocks disrupts the three dimensional stability of the foam and sometimes causes the foam to warp slightly.  This is not a problem.  The foam cores and shucks will fit back together and be as perfectly straight as they were when they were cut, when they are placed on a flat table and weighted down, like when you apply the sheeting.  What is important is that when you skin the wings that it is done on a flat surface with the shuck marked TOP as the top. The weight or Vacuum bag will hold everything flat. When we cut wing we have about 70lbs (32 kg) on the block of foam.  Remember that the foam has very little strength and almost no stiffness. The balsa skins are strong and stiff, which always dominates and carries the load. Once the skins are bonded in a flat position the wing will always be flat. Think about it from a fiber perspective. Fiberglass cloth is limp and has no structure. Epoxy DOES NOT react with fiberglass yet when added together you get a stiff structural part. The reason is that the epoxy holds the fiber "in place". The combination gives you the structural part. The same is true for the wing. The components are all limp and non structural until added together. Have you ever worried that since the fiberglass cloth is limp that your fuse ! will warp? The wing core has one purpose:  to give the skins something to glue to to "hold them in place". If it is done flat it will always be flat, just like if a fuse is molded straight it will always be straight (unless heated to soften the epoxy), with a former or equivalent to hold the shape "in place".

The foam itself has very little structural integrity.  As you build internal structure, like spars, it's important to hold the foam straight in the shucks.  It will bend easily into shape.  These spars will begin to add structural integrity.  Adding the skins is the final step.  It will hold everything in place.  A small warp or wave in the foam is insignificant as long as the foam was cut correctly.  The foam used to cut our wings is virgin foam that is cut into panels and aged about 3 months so their stresses should have stabilized.  The airfoils are cut on a very flat table and weights on top.  Returning the wings/shucks to this condition is the only way to verify that the wings were cut correctly.  Holding a core in free air after it has set in your garage/house for a while may show a bow or warp, but this is due to transient moisture and the stress relief that occurred when the wings were cut.  Once returned to the shucks and weighted down they will be restored to their proper shape.  The stressed balsa skin adds tremendous strength and stiffness and completely overcomes the wimpy stresses of the foam.  By the way, even after sheeting a perfectly flat piece of foam, it can change its moisture content due to atmospheric conditions.  You never see any change in your finished wings though.  This is because once trapped in the skins, their "warping force" is far too weak to change the structure.