Always keep them for later use. Ancient Knowledge must be used in the last battles (including the boss battles), where the XP is the most so that you can get the most out of them. +10 Atk/Def, +30 rage/mana scrolls are very useful in boss battles and castle sieges.
Scrolls like Song of the Wind, Death Call, Call Collosus will give you a full troop, so it's up to you to decide when do you need some extra firepower.
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