I'd like to Think that I am. I have all fourstars champed except Black Panther and Valkyrie, and I can Always level up new fours as soon as I get the covers. I am rerostering my threestar dupes and I am over the hump there - 37 rerostered already, with 10 still at base level due to a lack of covers. When I get the covers, I can champ them too.
The fivestars are Another matter. I am up to 10 fivestar champions, and Archangel has 13 covers and Black Bolt 12. The others range from 8 covers (Goblin) to just 1 (several). So there's no real urgency to level any of them, but if I wanted to I could sink a lot of ISO into them.
Overall though it feels like I definitely earn more ISO than I have reason to spend.