With the news that old faction gear and weapons will be added to the world loot pools, it might be smart to not decrypt any Prime Engrams you get until next season. If that gear is indeed added next season, the item you engram decrypts into doesn’t get chosen until you go to Rahool. So give yourself a few early chances at whatever updated faction loot ends up in the loot pool! I doubt anyone needs the engrams for power level right now, and the worst that happens is it doesn’t work and you’re in the same boat as if you turned them in now.
Only reason I’d see for cashing them in now is if you are still hunting for something specific from the current loot pool. Unless Bungie removes some item