Just to be clear - when most of us refer to "starter" champs, what we mean is the four options you get when you create your account: Kael, Athel, Elhain, or Galek. Barring some very lucky early draws, you're likely to get the most value out of bringing that specific starter champ you chose to 60.
The login champs, on the other hand, are a very different set. From those, Khatun, Scyl, and potentially Ghrush are worth leveling. Scyl almost certainly to 60, Khatun possibly just to 50, and Ghrush depends on who you have when you get him.
More importantly, though, a lesson you are well-advised to learn as early as possible is - never invest resources in *anyone* without a specific reason; that means, no leveling, no ascending, and especially, *no booking*. Just because you got some shiny new epic, or even legendary champion, doesn't mean you should start leveling them. Patience is key in this game. Leveling food is never a waste. Leveling a random champ could very well set you back considerably, especially if you start ranking them up.