You can't champion all the characters, it's just too much ISO. But definitely you want to when you can, because of the Legendary Tokens at 167 and just free goodies from there on out. But if you try to max all the 3-stars, by the time you earn enough to do so you'll have a bunch of maxed four-stars who you haven't been leveling up and are forced to toss covers.
My IW is level 170 and 13 covers. I've tossed 3 extra Blue covers in the past week since I know I am not going to get the 200K ISO before they expire. Which is crazy because that's a lot of "lucky" 4-star pulls, oh if only they were for anyone else..
Anyways I would suggest championing who you can, but NOT everyone. It's just too iso heavy. I envy the guys who never levelled up and are sitting on a fat stack of ISO to choose exactly who they want to progress, but won't feel bad when they pop their LTs right afterward and then realize they are 250K short of Champing a 4-star with a couple covers waiting.