Why do this? Why would it be especially important for a DD turntable? TT's are most often guilty of putting noise back into the AC supply for the rest of the system, so it would be prudent to isolate the AC supply to the TT. Likewise, shielding might be worthwhile, to prevent RFI from contaminating the supply to the TT. So, I could imagine that a shielded AC cord would sound different from an unshielded one, in a high RFI location.
