I fill first - before I punch. I don't like going through the trouble of punching and setting up a tank only to discover I have a DOA carto. So I fill it normally, vape it, and once I've decided it's good, then I punch it w/ a saddle valve and slide it into the tank.
Probably not recommended if you're slotting w/ a dremel, since wet filler would more likely collect more metal dust.