Using Spells Effectively in Elder Sign
The way spells function was actually amended in the Errata and subsequent mobile game, such that spells are instead cast after a player's roll - so you cast them only when you need them.
Full text from the errata:

Spells that hold dice are cast after a player’s roll, not before. The player looks at the roll, then, if he wishes to save a particular die, casts the spell and places the die on the spell. The ‘held’ die remains on the spell card until used to complete a task or until removed (as described below).
• Dice may have their results modified by items/abilities before being placed on a spell, but not afterwards.
• Before any roll, a player may remove some or all of the dice held on spells in order to roll them. Any spell that has no dice remaining on it is discarded. New dice cannot be added to a spell to replace those that were removed from it.
• If a die becomes locked while it is on a spell, the die is removed from the spell and locked. If the spell has no dice remaining on it, the spell is discarded.