I'd say in the middle of ToB, right before you fight Abazigal and Sendai, helps make the game a bit longer considering there's not that much to it. But if you have the
Tactics Mod with all of the improved bosses, I'd say do it before you go to Suldenessular so you stand a really good chance against Irenicus, but make sure you're prepared so you don't get killed in the early going.