I've always been panel first, but story beats/storyboard first-er. My panels have to hit each storybeat in some way and unless a scene is specific about dialogue, I won't worry about it till the very end. I'll make notes on what I want a character/characters to say, but never their entire dialogue. Even if the words I use can't be understood (language barrier, inability to read or just skimming through), the panels and visuals should still convey the story. Plus, it takes me 5x longer to draw out the panels, so naturally I want to get this stage done first. Dialogue is one of the last things I work/settle on in my production.