I've always thought that shielding effects tone, most audible in the hf's. People like to argue about this...I don't want to start a flame war, but I do believe that it effects tone. I've talked to others who feel the same way.
A part of the sound of those pre cbs p basses is the thin metal pick guard shield, as is the sound of the late 50's P's with anodized aluminum guards. maybe putting the tape on the bottom of the guard has the same effect, but simply installing the one that fits the pick guard is easier to do.
I bought my last fender p bass pg shield on ebay for $11.95 about a month ago, the dealer had quite a few in stock. Pretty much the same price as a shielding kit?