By 'dialogue first', do you mean writing the dialogue into the panel and deciding where the balloons will be, or like... actually writing the dialogue?
I work from a written script. Sometimes, as I'm thumbnailing panels, I'll add in extra snippets here and there... usually jokes, and offhand quips. But 90% of what's in my comic came from my script.
I'm often told my dialogue flows really well, and the character voices sound very natural. I attribute that to two things; working from a script, and making sure I read dialogue out loud while I'm scripting. Working without a script seems like it'd weaken the story, as it's far more difficult to plan out exposition, characterisation and so-on if you're flying by wire the whole time. Even pantser novelists edit their work before release, you can't do that with a serialised comic.
Where drawing my comic is concerned, I don't pay too much attention to where the dialogue will go while I'm thumbnailing/sketching. (A huge perk of working in the vertical format is that dialogue tends to float between panels rather than being contained within them, so it doesn't matter. If I were working in page format, I'd pay far more attention to it.) Once I have all of my sketches, I add the dialogue from my script to ensure there's enough space between panels for everything, and that the eye flows nicely between bubbles.