How are the shielding tubs grounded? That's my question about them... I honestly probably would have used them had I known about them when I rebuilt my guitar, but I am not sure how to ground them.
What I did with the paint was, I drilled a screw into the body, and covered that with the paint, and soldered that to one of the pots- the volume, as I recall. Whatever the other main ground was soldered to. From there, the interference should be attracted to the shielding, and find the path of least resistance by running harmlessly to the ground. If there was no connection to the ground, then nothing would be accomplished, really. How do you do that otherwise?
Honestly, "shielding" is sort of a misnomer, you aren't keeping anything out, you are making electrical interference go where it won't be found by the pickups and then amplified.