Well...
Covers don't NEED to be soldered on humbucker pups. It doesn't hurt anything to solder them on, for sure. But they already "buck" any hum, so soldered/grounded covers don't really do anything to help that. Possibly to prevent any microphonics from a metal cover vibrating against a metal baseplate, but not really for grounding to prevent electrical interferences. If that were the case then every single coil pup should have metal covers soldered on.
I have soldered metal covers on humbuckers and I have also just used double stick foam tape. I haven't noticed any difference at all. At first thought you would think that, theoretically, it would be important to solder them on to ground the covers. But why would that be so important on humbuckers and not single coils (most single coil covers are plastic)?!
Just remember, you risk some damage when you solder them on. Anytime you take a soldering iron in hand you risk some damage, especially if not done properly.