When it comes to this, it all depends on what kind of player you want to be. Use them on PvP heroes if you want to be a PvP player. Build up heroes for Coop if you want to grind for hero frags, or others raids. If you want to earn more heronium, to buy other frags, build up Matador, Mavin, Surge, and Cinder. (Daily Rewards) I would probably stay away from building up the 7 stars. Not because there bad, but in the end, there just like any hero who is seven stars. I would definitely build up 5 stars though, for the most part, all of them are good. Building the lowest of your lineup isn’t a bad idea either. If you haven’t already done so, I would build all your weaker heroes. Get all of them all above Gold. You could also build up the bonus faction heroes for bounty, if you want to deal heavy damage. In the end though, you can do some many different things with heronium. It’s your choice. Hope this helps a bit.