Is shielding a guitar worth it?
Power transformers and street lights can also cause interference. A shielded guitar cavity will go a long way towards preventing the interference from reaching your signal. Shielded wire leaves your volume and tone pots exposed, so shielding the cavity does a better job of protecting your guitar.
Does a P Bass need shielding?
IME it's not necessary on a P bass as long as everything is wired correctly, bridge grounded correctly, good quality pots with their cases grounded to a common point, etc. But it wouldn't hurt anything to do it, either.