Properly placed, a pickguard is more than decoration, it's protection against scratches (or in Willie Nelson's case, holes) in the top of the guitar. I would think it has to affect the sound to some degree. It may be a very small effect with a thin pickguard. It seems obvious that if the pickguard covered the whole top it would change things tremendously. It would be interesting to see if anyone has studied how much difference it makes in the sound.