The question is would it change where you could tell the difference? I doubt very much unless you have the hearing of Superman
Did Superman have super hearing

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The strongest reason for the people to retain the right to keep and bear arms is, as a last resort, to protect themselves against tyranny in government.

Thomas Jefferson

I would imagine they would stop the top of the acoustic from vibrating/resonating, so I could see it making a huge difference on those guitars. On an electric guitar the effect has to be infinitesimal. Unless of coarse you use a ferrous material, that affects the magnetic fields of the pickups.

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I'm not an expert, but I play one on the internet.

I changed the metal pickguard on my classic vibe 50s Squier.It is the white model and it came with a gold pickguard which is ok but nothing too gorgeous in my opinion.I had a mint green pickguard which looks killer on the white so I installed that instead.The tone did change somewhat.I think electronically,the metal pickguard has more capacitance and therefore dampens some of the highs.With the plastic pickguard,there is much less of that electronic dampening happening and I hear that more bell-like Strat tone out the same guitar.

You should ask the same guys who think taking off the tremelo spring cavity cover affects your sound!!! Oh yeah, and wood "breathes" tooo!!!
(that should stir up those fairies!!!)