Pretty much, but you have to actually squeeze your abs super hard. Like when you were a kid you tried to turn your face red by pushing all the blood into it. But yes, most DEFINITELY a belly breathe not a chest breath.
I prefer to get the breathe before I dip to grip the bar, but I don't spend very much time at all after I grab the bar before starting the pull (maybe...half a second?). If you spend a lot of time with your hands on the bar you might want to take a breathe after you grip when you're ready to start pulling.
Basically whatever nighthawkz said, do what feels best.