I have never tanked and I usually fight until the last match of the season. But for what it is worth, I don't think there is any reason for a L1-3 to tank. You are almost always up against only 1 or 2 active players. There rest are abandoned teams. You can collect more trophies in L1-3 without tanking, unless of course you play with your friends. That is harder to win.
Another thing to consider is whether you want to invest in green packs, which would cost you tokens, to train your fast trainers, if you have any. You would need abt 300 greens and an abundance of reds to train fast trainer.