You bring up a different aspect that I have always thought important.
Straight (not warped) is very important to build structures that are true (not crooked with them selves).
Level on the other hand is only important if for instance your using incidence meters to set up the relation of the wing to the stab. I use incidence meters on the wings and then measure to the front and rear of the stab to set it relative to the wing. In this case your surface needs to be both straight AND level. Hope I'm not getting more involved here than what your original question was.