Initially I didn't and just focussed on a strong six. As ai played more and more, I ended up with a dozen characters that were all level one, some with 3, 4 or even 5 stars that had never been touched. As I started playing the galactic war element, I found I needed a much bigger squad than six though and have started to work on the other characters too. The secondary set(s) are still weak in comparison as I don't have the gems etc to upgrade them all and as you get stronger at the top end they get expensive to keep up with your level, so I try to balance what I win across the mix to keep a decent squad overall really.