We use single edged razor blades or an X-acto will do too.
Older Gibsons are tricky at times because they were finished with the necks on.
Older Guilds are usually the worst though also because of being finished with the necks on but with the added issues of a poor heel shape for the neck reset process and the tendency that Guild had to use too much glue AND apply it to the heel cheeks and subsequently the sides instead of judicious use of glue on the dovetail only. Many shops charge more to do old Guilds and we usually do too because of the complexity and then the need for more than an average amount of touch-up work.
By the way George good on ya for doing resets and doing several of them too. Resets are very much something that require skills that include not only many repair practices but knowledge of fitting necks, neck angle, finishing, etc.
FYI there is thinking among the pros who do this stuff for a living that really any neck reset should be accompianied by a fret dress too since we are changing the neck angle, once again, and that has ramifications for the fret plain including the extension.
For us a reset usually, usually always, includes a fret dress and at times it's a good time to address any need for a refret too.
Currently we have five guitars in the shop for resets and in various phases of same. Depending on the instrument a reset can be pretty easy or something that one has to get on the floor and wrestle with a bit....
We often batch resets since we do them often. It's also a good time to do them in the winter when the added RH from the steamer has other beneficial uses.