[Arran]

I don't know if it will actually cause the glass to crack on a hot running tube, but one problem cyanoacrylate glues like superglue, crazyglue, and other brands have is that it tends to crystallize after a time and comes apart, and it doesn't like heat. Epoxy may or may not like heat either, depending on the type, but it doesn't tend to break down with age. The Permatex is somewhat heat resistant, and it's made for bonding glass to metal, so I think that it would be preferable for grid caps for sure, though I have used epoxy for this in the past as well. If you have to take the base off of a tube to resolder pins, such as with a 01A/201A, or a #26, or a #45, then I've found it works well to re-cement the base onto a glass tube, after you've cleaned out the old putty with lacquer thinner of course.
Regards
Arran