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It depends on how easy it is for you to say "no." It's very easy to created overpowered spells unless all you're doing something like taking fireball and changing it to cold damage.
I would avoid it unless the creation of the spell was important to the plot, at which point you don't even need it to be a real spell anyway.
At my game all I allow is swapping elemental damage types (fire, cold, electric, acid) for other elemental damage types. But I don't require spell research for them, they're just new pre-existing spells that can be selected.