Only people who can answer this question are the people in charge of the game because we dont know when kage treasure is coming :( AO is really good for fire he is probably better than sage naruto atm. Well worth it.
Now answer me : 1. Do you win sage battlefield and get reward from it often? 2. Do you win all survival trial everyday? 3. Do you win all matsuri rewards everyweek? If all the answers are YES. Then don't invest anything. The boredom mentality while using same team all over again will make you want to invest into something you don't actually need and it will hurt you more than it help in long run. As long as you can fight your boredom and the urge to waste your resource on unnecessary ninja you will be fine.
If your answers are NO then go ahead and do it. But aware that treasure pull are random, nothing garuatee you will get the ninja you want. People recommend Sage because it have the smallest ninja's pool.