Do humbucker covers have to be caulked/sealed/waxed?

If you have a low gain, clean tone then it may be fine. Otherwise there may be an increased tendency to microphonic howl, which can be pretty miserable.

Thank you both!!  

The metal cover can cause a pickup to become microphonic or oscillate--even when it's made from non-ferrous material.

An easy way to re-pot the metal cover onto the pickup's bobbin is to place a strip of tape over the holes on the metal cover and lay it face down on some cardboard. Use a cheese grater to grate some paraffin wax (an ounce is about right) and put that into the cover. Use a heat gun to gently melt the wax in the cover, and then quickly press the pickup into the melted wax.

Be careful because the hot wax may gush out and get on your fingies and that will hurt.

Carefully scrape off any wax from the edges and re-solder the ground connections between the cover and base plate. Flip the pickup over and peel the tape off. Give the metal cover a good scrub with an old towel to remove any wax from the surface.

You're probably fine.

Fun fact: the wax potting process was initially patented by Fender. It was for microphonics, longevity, and differences in tone were acknowledged even then. As mentioned, it was intended for the coil (metal covers being rare on Fenders), and that is why it needs to get into every nook and cranny, because that wire is so fine and tight.

The necessity for wax potting in relation to the cover itself will depend on how well the cover fits. Poorly fitted or poorly attached covers will be far more likely to give you grief than one lacking potting. Usually just having enough to be a slight mute is enough, a piece of tape on the inside of the cover is a common solution. The caulk is likely more than enough if the cover seated reasonably well and is affixed to the baseplate.

I knew a guitarist who was very much a high gain half stack metal guy, and one of his guitars had Seth Lovers - covers and no wax at all. He said on occasion they'd start going a little nuts, but if he put his hand on the pickup it quieted down and he was fine. It was rare enough that it didn't hinder him using the guitar regularly on gigs. On the other hand, on the (somewhat rare) occasion that the solder joint breaks on a pickup and the baseplate separates from the bottom of the cover, you can get squealing even from moderate overdrive and volume.

So...

Yes, probably, maybe, no, depends

😂