I did it all flight instructing, mostly Cessnas, a little bit in a Duchess, all of it VFR. It doesn't really matter- as long as you meet the 135 mins (or are close enough to meet them by your checkride), and can pass a basic sim ride during your interview, you should be good to go. If you can build time faster by flying for another 135 operator, that's cool. Some solid IFR experience will definitely help, but it's not necessary- I had less than 10 hours of actual when I interviewed. I got more than that my first week flying the line.